192 DISSECTION OF THE DOG 



occupying the posterior part of the cavity of the eyeball, and intervening 

 between the crystalline lens and the retina. In front, adaption to the lens 

 necessitates the presence of a concavity, the hyaloid fossa (fossa hyaloidea). 

 The rest of the surface of the body is convex and applied to the retina (optic 

 and ciliary parts), but is not in any way connected with this tunic of the 

 eyeball, except at the entrance of the optic nerve. 



The surface of the vitreous body is covered by the delicate hyaloid membrane 

 (membrana hyaloidea), which, over the ciliary body, is thickened by the acces- 

 sion of radial fibres. This thickened part of the membrane is known as 

 the ciliary zonule (zonula ciliaris), and is firmly adherent to the ciliary processes 

 to which it is adapted by a series of radial folds and grooves. Close to the 

 margin or equator of the crystalline lens the zonule splits into two layers. 

 The thinner of the two lines the hyaloid fossa ; while the much thicker anterior 

 layer is connected with the capsule of the lens, and thus forms its suspensory 

 ligament. 



Running through the vitreous body from the optic papilla to the back 

 of the lens is the minute hyaloid canal (canalis hyaloideus) marking the line 

 of the embryonic hyaloid artery (a. hyaloidea), a branch of the central artery 

 of the retina. The canal cannot be detected in adult material unless the 

 vitreous body has been stained. 



Spatia zonularia. — If a blow-pipe be introduced through the suspensory 

 ligament, a varicose canal surrounding the equator of the lens can be inflated. 

 This is formed by the zonular spaces, bounded in front by the ■ suspensory 

 ligament and behind by the hyaloid membrane covering the vitreous body. 



Dissection. — Separate the lens from the vitreous body by cutting through 

 the suspensory ligament. 



Lens crystallina. — The transparent and biconvex crystalline lens lies 

 between the iris and the vitreous body. It should be noted that its surfaces 

 in the dog are not so convex as in other domestic mammals. A zone 

 between the extreme periphery and the centre of the anterior surface (facies 

 anterior lentis) is in contact with the iris ; whereas a variable amount of the 

 central part of the surface coincides with the pupil. The posterior surface 

 (facies posterior lentis) has a greater curve than the anterior and is received 

 into the hyaloid fossa of the vitreous body. The central points of the surfaces 

 are known as the anterior and posterior poles of the lens (polus anterior lentis 

 et polus posterior lentis). Joining the two poles is the axis of the lens (axis 

 lentis). In accordance with the comparative flatness of the surfaces, the 

 margin or equator (aequator lentis) is moderately sharp. 



If the lens be held up to the light, fine radiating lines may be detected 

 spreading out from the central point of each surface. These indicate the 

 planes of contact of the ends of the fibres (fibrae lentis) which compose its 

 intimate structure. Covering the surface of the lens is a homogeneous 



