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Abstract
Comparative morphological studies on the vascular tunic of the eyeball in some domestic animals
SUMMARY AND CONCLUSIONS
The present work was carried out on 16 eyeballs from each of the following animals: donkeys (Equus asinus), buffaloes (Bus bubalis), camels (Camelsus dromedarius) and dogs (Canis canis) to elucidate the gross anatomical, light microscopical and scanning electron microscopical features of the vascular tunic. In addition morphometric data were carried out on the latter tunic in all animals under study.
The absolute volume of the eyeball is variable among studied animals. The camels have the smallest eyeball among the large animals under study. It can be postulated that the size of the eyeball of an animal is inversely related to the amount of light in the surrounding environment.
The volume percentage of both vascular tunic and aqueous humor to that of the eyeball displays concomitant variations in the studied animals. Their highest percentages are seen in dogs followed by donkeys and camels, however their lowest percentages are recorded in buffaloes.
The position of the Tapetum lucidum in an animal is affected by the visual horizon in that animal. The horizontal base line of the tapetal area just touches the optic disc in donkeys, encloses it partially in dogs and completely in buffaloes. This means that the Tapetum lucidum is situated more dorsally in donkeys than in dogs and buffaloes respectively, which seems to be related to the posture of the head and consequently the visual horizone in these animals.
The Tapetum lucidum is either fibrous as seen in donkeys and buffaloes or cellular as observed in dogs consisting of variable layers of well ordered collagen bundles or tapetal cells respectively. The latter structures are of almost constant size and separated by almost constant spaces. The number of tapetal layers varies from animal to another. The Tapetum cellulosum of dogs demonstrates the presence of more layers than those of donkeys and buffaloes, that may indicate a higher reflectance in the former animal.
The camel shows no Tapetum lucidum grossly, however microscopically its central part demonstrates the presence of many layers of thin collagen bundles. It seems that the camels has a weakly developed, functionless Tapetum lucidum fibrosum, which is blocked by the covering pigmented retinal epithelium. This could be attributed to the bright environment in which the camels normally lives, where adequate light is available.
There are few occasional smooth muscle cells in the choroid of the dogs. These cells may counteract the contraction of the ciliary muscle in this animal.
The volume percentage of both ciliary body and aqueous humor to that of the eyeball displays concomitant variations in the studied animals. Their highest percentages are seen in the dogs followed by the donkeys and the camels, but their lowest percentages are recorded in the buffaloes. Similar variations are once more recorded for the relative surface area of the ciliary body to the total area of the vascular tunic. This means that animals with well-developed ciliary body have high relative volume of the aqueous humor in their eyeballs.
The average number and shape of ciliary processes vary in the different examined animals. They are about 95 in donkeys, 100 in buffaloes, 98 in camels and 75 in dogs. They are high in both donkeys and dogs with some secondary processes, but they are relatively low in buffaloes and camels. The anterior ends of the ciliary processes extend over the base of the iris in all examined animals except dogs, where the processes terminate abruptly.
The medial length of the ciliary processes is shorter than their lateral length in donkeys, buffaloes and camels in comparison to their nearly similar contralateral length in the dogs. The medially short ciliary processes, in examined herbivorous animals (donkeys, buffaloes and camels), logically shortens the interpupillary distance that may overcome the well-known narrow field of binuclear vision in these animals.
The suspensory ligaments of the lens (zonular fibers) are well developed in donkeys and appear in the form of a closely adherening network covering the ciliary processes, which may indicate that this animal has the strongest suspensory zonular apparatus.
The ciliary muscle is best developed in dogs, followed by donkeys, camels and buffaloes. It can be concluded that the dogs has the best ability for accommodation of near vision. The latter conclusion may facilitate the nocturnal life and predatory feeding habits of this animal. All studied herbivorous animals (donkeys, buffaloes and camels) have weakly developed ciliary muscle. This coincides with the limited accommodative ability in these animals. Donkeys, however, has slightly stronger ciliary muscle than camels and buffaloes indicating that its accommodation may be slightly better than the latter animals.
The iris is the most anterior portion of the uvea. It constitutes a variable volume proportion (in relation to the eyeball) and relative surface area (in relation to the total surface area of the vascular tunic) in the different studied animals. The highest volume proportion is recorded in dogs followed by buffaloes, camels and donkeys. Its relative plane is the highest in camels followed by donkeys, dogs and buffaloes. The breadth of the iris varies topographically in the studied animals. In buffaloes and camels, the dorsal and ventral breadths are larger than the medial and lateral ones, while in donkeys and dogs the breadth of the iris is nearly uniform in all directions. Accordingly, the vertical and horizontal diameters of the pupil vary in different studied animals being transversely oval in buffaloes and camels, but it is semicircular in donkeys and typically circular in dogs.
With exception of the dogs, the pupillary border of the iris carries several black masses (Corpora nigra or Granula iridica) of variable size specially on the upper side. These masses are large in camels, small in donkeys and buffaloes.
The anterior and posterior surfaces of the iris are studded by circular and longitudinal folds indicating the arrangement of the constrictor and dilator pupillary muscles respectively. The longitudinal folds are thrown into numerous transverse ridges, which are clear in donkeys and dogs (animals with strong dilator muscles) contrasting the picture in buffaloes and camels.
The iris is composed of fibrovascular stroma hosting the constrictor and dilator pupillary muscles and is covered anteriorly by thin epithelial layer, but posteriorly, it is covered by a thick pigmented epithelium. The constrictor muscle occupies variable breadth in relation to that of the iris indicating its myotic strength. The relative thickness of the dilator (in relation to that of the iris), however, varies in the studied animals that indicates also its medriatic efficiency.
The camels has the weakest dilator muscle in combination with a relatively strong constrictor muscle that is a prerequisite of a strong miosis, this character seems to be suitable for the bright environment of the camels’s habitat. The dogs has the strongest dilator and constrictor muscles of the iris; this likely enables this animal for a maximum excursion. The dogs as a nocturnal animal may be equipped with strong dilator pupillary muscle capable of maximum pupillary dilatation during periods of reduced light, which allows maximal stimulation of retinal photoreceptors.
The anterior surface of the iris is covered by a single layer of flat epithelial cells, while the iris is covered posteriorly by two layers of pigmented epithelial cells continuous with the two layers of retinal epithelium, which line the ciliary body, the deep layer is transformed to the dilator pupillary muscle. The posterior epithelium demonstrates apically placed vacuolar aggregations, which are tremendous in camels. The latter vacuoles seem to be extensions of the Corpora nigra, which have the same structural appearance.