Theory: Morphological and functional features of the structure of the Zonula Lens Fibers as a key executive link in the mechanism of the human eye accommodation

Purpose: Assess morphophysiological and functional features of the structure of the zonula lens fi bers on the basis of an in-depth analysis consistent with the laws of mechanics. Defi ne the purpose and scope of each portion of the zonula lens fi bers. To analyze the results of the research of studies on the structure of the zonula of Zinn in the different authors of the last two centuries. Identify misconceptions about the work of this executive mechanism of accommodation in different phases of accommodation, this includes the uneven accommodation in different meridians. Determine the incorrectness of those theories of accommodation, which are based on erroneous ideas about the functioning of different portions of the zonula fi bers. Methods: Theoretical analysis based on the laws of mechanics and assessment of the results of interdisciplinary research by different authors in the fi eld of structural and functional features of the Zinn's zonule of the lens in different phases of accommodation. Results: The lens’s ciliary belt has been found to have three major portions of fi ber that perform different functions. Powerful anterior portion of fi bers is attached to the anterior surface of the lens bag in a segmented mannered a powerful posterior portion of fi bers is attached to the posterior surface of the lens evenly around the circumference and like a cobweb covering the surface of the vitreous chamber. Both these fi laments in the eye don’t cross and in all phases of accommodation remain Review Article Theory: Morphological and functional features of the structure of the Zonula Lens Fibers as a key executive link in the mechanism of the human eye accommodation Ivan N Koshits1, Olga V Svetlova2*, Maksat B Egemberdiev3, Marina G Guseva4, Felix N Makarov5 and Nikolas M Roselo Kesada6 1Biomechanics, General Director, Petercom-Networks / Management Systems Consulting Group, Cl. Corp., Saint-Petersburg, Russia 2Professor, Department of the Ophthalmology North-Western State Medical University named after I.I. Mechnikov, Saint-Petersburg, Russia 3Chief of the Department of Ophthalmology, Chuy Region United Hospital, Bishkek, Kyrgyzstan 4Ophthalmologist, Optometrist Medical Diagnostic Center of the Public company, Vodokanal of SaintPetersburg, Saint-Petersburg, Russia 5Professor, Chief of the laboratory of Neuro Morphology of the Pavlov Institute of Physiology of the Russian Academy of Sciences, Saint-Petersburg, Russia 6Postgraduate student of the Department of ophthalmology, Pirogov Russian National Research Medical University, Moscow, Russia Received: 07 July, 2020 Accepted: 21 July, 2020 Published: 22 July, 2020 *Corresponding author: Olga V Svetlova, Professor, Department of the Ophthalmology, North-Western State Medical University named after I.I. Mechnikov, Kirochnaya ul. 41, Saint-Petersburg, 191015, Russia, Tel: +7 921 9724900; E-mail:


Introduction
Although accommodation determines high-quality vision and is one of the most important sensory systems in humans, there is still no generally accepted theory of accommodation in ophthalmology. Accommodation standards are not fully developed and not introduced into clinical practice. This is due to the fact that over the past two centuries, many eye researchers, when explaining the results of morphological studies of the executive structures of the accommodation mechanism, did not fully take into account the laws of mechanics. The widespread use of the latest methods for studying the structures of the human eye based on high-resolution electron and ultrasound microscopy did not improve, but, on the contrary, confused the idea of the accommodation mechanism even more, because in the morpho-physiological interpretation of the structural features of the structure of the Zonula Fibers (ZF), the laws of mechanics were not always respected again. Paradoxically, even some leading ophthalmologists so far have not had a clear idea of the exact location and fi xing in each portion of the ZF, although an adequate idea of the operation of the main intraocular systems: accommodation and outfl ow of aqueous humor depends on understanding this issue. An interdisciplinary approach for assessing morphological results tense: in full distance -maximum, and when you look close, it's minimal. These functional features of two powerful portions of the zonula of Zinn are fully consistent with Helmholtz's vision and allows them to hold the lens relative to the optical axis of the eye even under high dynamic or weightless conditions. These two portions of fi ber are force-bearing and constantly press the lens against the vitreous chamber, fi xing its normal position relative to the optical axis. It is characteristic that both these powerful portions of fi bers pass in the direction of the choroid through the recesses between the processes of the ciliary muscle. It is important that both of these portions of fi bers are not attached to the ciliary muscle by force, but are only held by the thinnest outlet fi bers in the recesses between its processes. The anterior and posterior portions of the fi bers are attached forcefully to the anterior of the lens bag, and behind they are attached to the choroid in the ora serrata region. The segmental attachment of the anterior portion of the fi bers to the lens bag allows for uneven lens accommodation in different meridians. The third portion of ciliary zonula fi bers -cilio-equatorial fi bers, on the contrary, is not strong and is located evenly along the equator of the lens. Two rows of these thin fi bers on one side are attached in the form of a frenum evenly along the equator of the lens bag, and on the other side they are attached directly to the ciliary muscle in the recesses between its processes. The task of these fi bers is to uniformly collapse around the extreme periphery of the lens bag in the equator region to keep the lens masses near the optical axis, preventing them from shifting to the equator inside the lens bag during different phases of accommodation.
Conclusion: Analysis of the morphophysiological structure of the Zinn's zonule of the lens, adequate to the laws of mechanics, made it possible to deepen and expand the classical theory of lens accommodation H. Helmholtz -where the lens is the main, but not the only mechanism of accommodation in the human eye. It was found that the executive mechanisms in the lens accommodation are: ciliary muscle, zonula of Zinn, vitreous chamber and choroid. The choroid is a biological spring and passively stretches the ciliary muscle depending on the level of its active tone. In all phases of accommodation, the lens is pressed by a powerful anterior portion of fi bers to the hyaloid membrane of the vitreous chamber. At moments of external dynamic loads on the eye, the energy of inertial micro movements or the lens phacodonesis is transferred to the vitreous chamber, which dampens these vibrations due to the dispersion of this energy between its internal structures. These ideas are adequate to the laws of mechanics and exactly correspond to Helmholtz formulations: in all phases of accommodation, the anterior and posterior portions of the fi bers remain stretched and their tension only slightly relaxes when looking completely closer. The detected segmented attachment of the anterior portion of the fi bers to the anterior part of the lens capsule allows for uneven accommodation in different meridians to partially level the induced astigmatism of the optical system of the eye under different visual loads. The most important result was the understanding that the pressure in the lens is at maximum when viewed completely closer. This leads to the extension and rounding of the anterior and posterior mini lens of the lens and increase their refractive power for rays passing near the optical axis of the eye. On the contrary, when looking completely into the distance, the choroid stretches the anterior and posterior portions of the fi bers as much as possible, due to which the lens bag is maximally stretched, and the lens itself becomes fl atter.At this moment, the pressure in the lens is minimal, since the elastic capsule of the lens is stretched from the outside by the zonula fi bers and therefore compresses the intralenticular masses the weakest of all with less force. It is at this moment that the work of the cilio-equatorial fi bers is necessary in order to evenly squeeze the periphery of the lens along the entire equator and keep the lens masses in the center of the optical axis. The results also confi rmed the incorrectness of many theories of accommodation compared to Helmholtz's theory of accommodation. Especially incorrect seems the extravagant theory of accommodation R.A. Shachar.
using the simultaneous application of research methods in ophthalmology, physiology, biomechanics and control theory allows us to obtain "at the intersection of sciences" an adequate modern understanding of the accommodation mechanism. And this mechanism should be based on an in-depth understanding of the morphological, functional and design features of its key link -ZF ( Figure 1). Our i deas (1994)(1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005) [2][3][4][5][6][7][8] about the functional role of the vitreous chamber, which is a necessary executive element in all phases of accommodation, coincided with the opinion of a number of authors [9][10][11][12][13]. All of these authors also considered the vitreous chamber as a possible actuator to assist in the rounding of the lens capsule during near accommodation. As Citation: Koshits  we did before, Lütjen-Drecoll E, et al. [13], recently designated the vitreous chamber as a structure that is capable of damping lens vibrations. It is worth noting separately the views of D.J. Coleman (1970Coleman ( ,1986 [12,14], according to which the lens rests in an elastic suspension of zonular fi bers, the spatial architectonics of which ensures their functioning like a cablestayed chain line at suspension bridges ( Figure 2). power to explain the total accommodation volume observed in practice of 10 diopters or more; M. Tscherning (1898) [26] and A. Pfl ugk (1935) [27], who considered the force on the lens from the vitreous chamber to be more important in the accommodation mechanism than tension of ZF; J.W. Rohen (1969Rohen ( -1979 [28,29], who proposed his own interpretation of the functioning of various portions of ZF during Helmholtz accommodation and was widely used by ophthalmologists, but contrary to the laws of mechanics. R.A. Schachar (1992Schachar ( -2001 [30][31][32][33], who presented an interpretation of the accommodation mechanism opposite to H. Helmholtz and an interpretation of the functioning of ZF opposite to J.W. Rohen. Unfortunately, the views of R.A. Schachar today received active support from some eye researchers and ophthalmic surgeons, although the main hypotheses of his "new" theory of accommodation are clearly not adequate to the laws of mechanics, including his understanding of the role ofthe cilio-equatorial portion of fi bers (CEF) as leading in the act of accommodation [34,35].
In-depth studies of ZF, which we conducted over 20 years, allowed us to draw a number of important practical conclusions and to develop the H. Helmholtz accommodation theory, which is consistent with the laws of mechanics in all phases of accommodation.
The structure and functioning of the supporting apparatus of the lens Today, there are reliable studies of the ZF structure at the beginning and end of the 20th century, striking not only with the fundamental approach, but also with the depth of interpretation of the results. The use of high-resolution electron and ultrasound microscopes as a research tool made it possible to obtain unique micrographs of the ZF structure [13,28,29,36,[37][38][39][40][41][42].
However, in a number of cases, due to errors in the formulation of the research problem, as well as in the interpretation of the results, some authoritative researchers were signifi cantly confused with the initially correct ideas about the functional role of various portions of the fi bers of the ZF. This led to fundamental disagreements in understanding the accommodation mechanism in different ophthalmological schools, but also "ideologically" prepared the appearance of the "extravagant" theory of accommodation R.A. Schachar [32,33].
It was only possible to cut this truly "tight" knot recentlyafter conducting ZF structure studies by experts in the fi eld of ophthalmology, morphology, physiology, biomechanics, and control theory [35,38,39].
At the beginning of our research [43,44], we went the traditional way and calculated the dynamics of breakage of ZF during lens subluxation after concussion based on the scheme for fi xing the lens in the tendon of Zinn presented to us by the leading ophthalmologists of the USSR: this traditional scheme involves fi xing the lens (in terms of biomechanics) "in an  [14].
When looking closer, the vitreous chamber directionally supports the lens from behind and helps it round.
This idea is still used in a signifi cant number of research works in which various mechanical experiments were carried out to stretch the bag of the lens in the equatorial region.
On the basis of these experiments, fi nite element models were considered for calculating the shape of the lens and the functioning of the accommodation mechanism [15][16][17][18][19][20][21][22]. Helmholtz [24]. The theory of accommodation was not fi nally completed by H. Helmholtz and could not explain a number of clinical observations. Therefore, to date, various "new" theories of accommodation are proposed, among which it is necessary to especially highlight the results of the following researchers: Gullstrand (1909) [25], who introduced the theoretical concept of the lens nucleus with greater refractive Citation: Koshits  If we accept the scheme of fi xing the lens in the eye using only ZF structures and crossing its Anterior Portion of the Fibers (APF) and its Posterior Portion of (PPF), then with such a constructive design of the lens attachment in the eye, it turns out to be virtually unattached due to the "relaxation" of the ZF at the moments of accommodation in the near distance. And this should lead to his constant trembling (phacodonesis) or frequent subluxations, which is not observed in practice. This means that there must be another, more reliable mechanism for fi xing the lens in the eye without crossing the ZF ( Figure   3b), but to detect this mechanism, we needed a more in-depth understanding of the morphological and physiological features of the ZF structure.
The results of direct studies of the structure and functioning of ZF were unexpected [3][4][5][6][7][8]12,13,28,29,[35][36][37][38][39][40][41][42][43][44][45][46][47][48][49][50]. It should be noted the important role of Russian researchers [24,34,35,39,[43][44][45][46][47]51] in the study of this issue. In particular, it can be considered fi nally established that the powerful FPF, going from the lens bag to the ora cerrata, does not forcefully attach directly to the Ciliary Body (CB), but passes between its processes and is woven by force into the choroid in its dentate region lines. These relatively fl at "reins" fi bers are located in  [52]. In order to create an uneven tension of FPF segments in the required meridians, the ciliary muscle must provide contraction (or relaxation!) of its corresponding spatial portions in these meridians. Although we note that these our ideas contradict one of the persistent myths of ophthalmology: the ciliary muscle is supposedly "single and indivisible" and is unable to contract "in parts". But it is the biomechanical and structural features of the FPF that convince   We especially note that H. Helmholtz considered two states as the extreme phases of accommodation: "there is no accommodation" (looking completely into the distance with a relaxed ciliary muscle) and "there is accommodation" (looking completely or not completely close at the maximum or not maximum tonus of the muscle ciliary) [23,39]. These ideas of H. Helmholtz are completely adequate to the laws of mechanics and rightly deny the presence of active accommodation in the distance. A myth common today among ophthalmologists that there is "active accommodation in the distance" and supposedly clinical evidence of this phenomenon, does not stand up to scrutiny because it contradicts the laws of mechanics and control theory of systems with unstable dynamic equilibrium [38]. But also note that the ciliary muscle has an initial physiological tonus of "warming up" even when looking completely into the distance, when the ciliary muscle has not yet begun to stretch the choroid. This initial tonus of "warming up" is necessary for the ciliary muscle to shorten the transition time from passive to active accommodation [38].
In fact, we must admit that in the absence of accommodation the lens is "squeezed" between the "powerful" stretched FPF of the zonula of Zinn and additionally "tense" from the increasing pressure in the vitreous chamber by the surface of the Anterior hyaloid membrane (AGM). This response increase in the "rigidity" of the AGM due to an increase in pressure in the vitreous chamber occurs when MC relaxes when the lens moves posteriorly and is pressed into the vitreous chamber by the increased force from the FPF. This effect is the basis of our developed method for early diagnosis of OAG using HRT-II, which allows to objectively evaluate the mobility of the Optic disk (OD) by increasing the volume of excavation after MC relaxation with a short-acting mydriatic [55]. At this moment, the tone of the MC is minimal, and the choroid is not stretched   magnitude of the effort and the amount of work performed by it as an actuating element in the intraocular accommodation mechanism. Speaking in the language of biomechanics, such a design of the accommodation mechanism makes it possible for the MC not only to get tired and save the energy consumed by the eye as an organ, but also to minimize the transition time from passive accommodation when looking completely into active accommodation at medium and short distances.
In order to fully understand the whole scheme of the power fi xation of the lens in the eye, it is necessary to understand how the muscle ciliary is anchored in the eye. The question of attaching the MC inside the eye is not as simple as it seems. The traditional idea is that the back of the ciliary muscle is woven by its fi bers into the choroid a (quote) "In its anterior section, the Brücke longitudinal muscle is attached to the scleral spur and trabecular plates and ... according to histological examination, the anterior ends of the meridional and radial fi bers are attached to the scleral spur and trabecular plates" [51]. But this, unfortunately, is not quite the right idea.  [15] (Figure 8), as well as RA Shachar [32,33] (Figure 9) believe that CEF is the main executive link in the accommodation mechanism, and all other structures that hold the lens in the eye in the natural position for optical beams, are auxiliary.
Reducing the diameter of the ciliary ring when looking closer leads to relaxation of the cilio-equatorial fi bers, and the lens is able to round. When looking into the distance, the diameter of the ciliary ring increases, the CEF portion stretches and makes the lens fl at. FPF and PPF do not participate in the act of accommodation.
The R. A. Shachar's point of view on the role of cilioequatorial fi bers and his theory of accommodation are based on premises that do not fully correspond to the laws of mechanics [7,8,24,34,35,38]. He rejects all accommodation theories that existed before him and proposes a fundamentally different mechanism of accommodation. In his opinion, the tension of the CEF at the moment of relaxation of the ciliary muscle pulls and collapses the equatorial zone of the lens capsule, tightens its peripheral part and forces the lens masses to move to the central region of the lens bag to increase its thickness here.
FPF and PPF do not participate in the act of accommodation [24,38].   Note that modern ideas about the structure of ZZ were based mainly on the fi ndings of J.W. Rohen [28,29]. Until now, its results have not been questioned by most researchers. Although J.W. Rohen made one obvious mistake during the formulation of the problem: he began to study ZF, having previously separated the lens and PPF from the anterior hyaloid membrane of the vitreous chamber (VC), excluding VC, like H. Helmholtz, from possible participation in the act of accommodation. The need to fi nd an explanation for the arcuate curvature of FPF and PPF under tension (Figure 12) [29] further led J.W. Rohen to search for "tension fi bers" that fi x the passage of both FPF and PPF directly in the notches between CB processes. J.W. Rohen actually discovered a number of thin "defl ecting" fi bers holding FPF and PPF between the processes of the ciliary body and described by M. Salzman as accessory fi bers [1]. (2)  B. Accommodation closer: the active force (7, gray arrow) of the ciliary muscle (2) additionally pulls the anterior choroid line anteriorly, while reducing the tension of the fi bers of the zonula of Zinn and the tensile force (8, short white arrow) of the lens capsule, as a result of which the lens is rounded ; the force (9, the long gray arrow) from the side of the choroid stretched like a spring at this moment is maximum and equal to the sum of two forces: the maximum effort (7) of the ciliary muscle and the minimum tensile force (8, short white arrow) of the elastic lens capsule.

A. No accommodation: ciliary muscle
As J.W. Rochen discovered, the cilio-equatorial fi bers located along the equator of the lens were also somehow fi xed in the recesses of the ciliary body. These ideas led J.W. Rohen   Rohen, the ciliary muscle transmits its force to the choroidea only through tension fi bers, which, when looking into the distance, completely relax, and, accordingly, in no way can take part in damping the vibrations of the lens (Figure 13b).
According to J.W. Rohen's rather thin "tension fi bers" must transmit force from the elastic structures of the lens capsule through the powerful FPF to the ciliary body. But for this, the diameter of the "tension fi bers" and their number must be larger than that of the FPF, respectively. This is not confi rmed by electron micrographs: the "tension fi bers" are much thinner and quantitatively less than those of FPF.
Therefore, they cannot play the main role in the mechanism of accommodation, but, possibly, play an important, but auxiliary role. In particular, they keep the FPF bundles in the grooves between the processes of the ciliary body, dampen shock loads acting on the lens, and also pull the FPF along an arc, like a bowstring, so as not to cut the surface of the anterior hyaloid membrane (Figure 13a and 14 Thus, this spatial bending of FPFs is necessary so that they cannot "cut" into the AGM of the vitreous chamber in the absence of accommodation. According to the J.W. Rohen scheme, it is at this moment that the "tension fi bers" are not tense, but relaxed (Figure 14c). Although it is at this moment that the tension of these abducting fi bers should be maximum.
Note also that with such an action from the FPF and PPF, the lens must move posteriorly, and in order to hold it in place, a force response from the vitreous chamber is required. And

Biomechanical interaction of the elastic lens capsule with the fi bers of the zonula of Zinn in different phases of accommodation
The physiological nature of the pressure inside the lens and  in the vitreous chamber, different from the IOP, has not been understood by most ophthalmologists until now. Therefore, this issue needs to be discussed in detail. It is characteristic that most researchers by default did not include the elastic structures of the lens capsule in the actuating mechanisms of lens accommodation. However, as described in RF Fisher (1971) [63], S. Krag and Tr.T. Andreassen (2003) [64] the elastic properties of the lens bag allow not only to ensure its rounding when looking closer, but also create intralental pressure Inside the Lens Pressure (ILP). The physiological nature of the occurrence of pressure inside the lens and in the vitreous chamber, other than IOP, has not been understood by most ophthalmologists until today. Therefore, this issue should be discussed in detail.
Note that, just like aqueous humor, all structures inside the lens are incompressible and unable to change their volume when the pressure inside the lens capsule changes. From a biomechanical point of view, these structures physically behave normally like water. But here it should be clearly understood that pressure in the lens can arise only when its incompressible structures are compressed from the outside by the elastic capsule of the lens. This is precisely the picture we observe on the enucleated lens: it is rounded as much as possible due to the work of compression of its elastic outer capsule, when there is no constant stretching effect on it from the anterior and posterior portions of zonular fi bers. At this point, the pressure inside the lens of the ILP is at its maximum. It should be noted that similarly, IOP is created in the eye only due to external compression of its internal structures by a relatively rigid but elastic fi brous sheath [65].
An excellent illustration of the discussion of this problem is the recent numerous attempts to ensure the fi lling of the lens sac with an inert transparent biopolymer after extraction of cataract masses in order to restore its ability to accommodate. O. Nishi (1998,2007) [65,66] has made the furthest advance here. It is characteristic that when fi lling the sac of the lens with a biopolymer, the volume of which was almost equal to the removed volume of cataract masses, the ability of the lens to accommodate was not restored.
The lens also could not accommodate when fi lling his pouch at 45% or less of its presbyopic volume. But on the other hand, when the lens bag was fi lled with a biopolymer, the volume of which was 50-55% of the volume of the cataract lens, its ability to accommodate was restored! And the reasons for this were diffi cult for many researchers to understand.
However, from the point of view of mechanics, everything can be explained quite simply. The fact is that the presbyopic lens has its volume about 2 times larger than normal [67]. And this is due to the accumulated and not removed slags inside his bag. These still transparent slags begin to expand from the inside and stretch the lens capsule. In fact, the presbyopic lens is an overstretched and already inelastic shell fi lled with incompressible cataract masses. Therefore, fi lling the lens bag with biopolymer by 75-98% of its volume before the operation, we actually overstretch its capsule again in comparison with its normal volume, i.e. we artifi cially transform the lens into a presbyopic one. Therefore, fi lling the lens bag with biopolymer by 75-98% of its volume before the operation, we actually overstretch its capsule again in comparison with its normal volume, i.e.
we artifi cially transform the lens into a presbyopic one. At the same time, in the presbyopic lens, the fi brous structures of his bag are stretched as much as possible, the rigidity of the bag itself increases because of this, which leads to a response jump in ILP. The increased ILP makes it diffi cult for fresh aqueous humor to pass into the lens through the outer epithelium. In fact, the lens loses its ability to "breathe", i.e. loses the functional ability to micro volume fl uctuations. And this happens, in particular, because the pressure inside the presbyopic lens is higher than normal.
We have already found out that FPF and PPF, as well as the CEF portion, stretch the lens capsule as much as possible when there is no accommodation (looking completely into the distance). And the capsule of the lens is minimally stretched with the tension of accommodation (look completely close).
Moreover, the process of active accommodation itself takes much less time than the process of diffusion of aqueous humor through the epithelium of the anterior capsule into the lens. Therefore, the normal volume of the lens during accommodation remains practically constant [68].
But precisely when looking completely close up, the elastic capsule of the lens is less stretched by the fi bers of the zonule.
And this allows the lens bag to round and, due to the reduction of its elastic structures, to squeeze the incompressible lens masses from the outside as much as possible. Moreover, the ILP will be maximum. This leads to the stability of the anterior and posterior mini-lenses of the crystalline lens, increasing its refractive power for rays passing near the optical axis ( Figure   15). This important mechanism of additional accommodation is described in [69].
The increase in pressure in the rounded lens when looking up close causes the surface of the anterior and posterior minilens to stand.
On the contrary, when looking completely into the distance, the lens capsule will be maximally stretched by the zonule fi bers, which will reduce its ability to compress the lens masses due to its elasticity. And this will lead not only to a decrease in ILP, but also to the appearance of waviness (shrinking) of the tissue of the lens capsule in its equatorial part, when the tissue of the capsule in this area is not fully stretched. Because of this, it becomes necessary to exclude the overfl ow of lens masses into the unstretched (wrinkled) equatorial part of the lens sac in order to keep these masses near the optical axis. The function of keeping the lens masses along the optical axis of the lens when looking completely into the distance is performed by the cilio-equatorial fi bers.
As it has already become clear, the main question that must be answered when creating any theory of accommodation is the following: when the pressure inside the lens is maximum -when looking close or when looking into the distance?
Unfortunately, the traditional answer of most ophthalmologists is: "the maximum pressure inside the lens will be when looking into the distance, when the lens is fl attened" [see, for example, [70]. And this answer does not quite agree with the laws of mechanics. Apparently, this is an implicit error connected with such a seemingly correct idea: in moments of lack of accommodation, the lens becomes fl atter, since it is "compressed" by the ZZ fi bers.
And since the substance of the lens is incompressible, the pressure inside the lens should increase. With the external correctness of such an explanation, the main circumstance is not taken into account that during the initial transition from accommodative stress to relaxation of the ciliary muscle, microstretching of the lens sac and micro increase (!) of its internal volume occur. Let us explain this.
The inner volume of the lens bag when looking closer is minimal: the elastic capsule of the lens squeezes the intralenticular substance as much as possible, so the pressure inside the lens is maximum. In the transition from fully close gaze to full distance gaze, FPF and PPF begin to stretch the lens capsule, thereby increasing its internal volume. an increase in the internal volume of an elastic shell when an external tensile load is applied to it follows from the theory of shells and is described in a number of modern works [71][72][73]. Since the interfi ber intracranial substance does not behave like a gas, but like a liquid (that is, it cannot expand when pressure drops), then with a micro increase in the internal volume of the lens bag, the pressure in it when looking into the distance will be less than when looking close ( Figure 16) [38].
With a micro increase in the internal volume of the lens bag when looking into the distance, a shift towards the equator of the intra-lens masses can occur, which is equivalent to decentration of the lens as a whole. In our opinion, CEFs serve to prevent this potential bias. This is the essential role of the CEF in ensuring the accommodation process is normal. In case of lens subluxation, this mechanism does not normally work, since partially torn FPF or PPF do not provide stretching of the lens capsule in this meridian, which is confi rmed by the clinic: displacement of intracranial masses and an increase in the thickness of the entire lens in the area of breakage of ZZ fi bers was observed by O.V. Svetlova [2] and other researchers in the clinic with his subluxation [3]. Taking into account all the above, it is now clear how correct the widely used schemes for fi xing the lens in the eye with the cross-location of FPF and PPF should be ( Figure 17a) [76,77]. Therefore, many researchers today understand how important it is to observe adequate boundary conditions when performing mechanical calculations of the lens shape in various phases of accommodation [20,22,76,77].
It is worth noting that the correct scheme for fi xing the lens in the eye presented in Figure 17b is completely adequate to the laws of mechanics.
It should be noted that a generalized model of the accommodation mechanism that is fully adequate to the laws of mechanics was created at the interdisciplinary level in Russia in 1999 [5]. Its main difference from the models of accommodation mechanisms considered here by other authors is that it does not contradict all known clinical facts, evolutionarily develops the classical theory of accommodation by H. Helmholtz and allows to clearly defi ne the boundary conditions for carrying out specialized calculations of the interaction of intraocular structures in the process of accommodation. This latest knowledge made it possible to create a generalized classifi cation of all accommodation mechanisms in the human eye, as well as to develop and test the metabolic theory of adaptive myopia with ten-year clinical observations [78][79][80][81].
In general, the refi ned morphological structure of the ciliary girdle and the physiological characteristics of its main elements, consistent with the laws of mechanics, made it possible to clearly understand not only the functional purpose of each portion of the fi bers, but also to determine its key role The role of the cilio-equatorial fi bers of the zonula of Zinn is functionally auxiliary, and their morphological structure cannot serve as a suffi cient basis for ideas about the dominant role of these fi bers in the executive mechanism of accommodation.