EYE 277 



functional relations to the optic apparatus (Amphibia, Sauropsida). 

 Only in the higher Mammals (e.g. Primates) is the orbit almost 

 completely surrounded by bone and thus separated from the 

 masticatory muscles, and in this case the periorbita is more or less 

 closely applied to or fused with the orbital periosteum. 



The deep orbit, formed by the skull, serves as a further pro- 

 tection for the eye, in connection with which there are also certain 

 accessory structures, viz. : 



1. Eyelids (palpcbroe). 



2. Glandular organs. 



3. Muscles for moving the eye-ball. 



Thus the eye-ball is formed of a series of concentric layers, 

 which are called from within outwards retina (sensitive and 

 nervous layer), choroid and iris (vascular layer), and sclerotic and 

 cornea (skeletal layer). The first corresponds with the nervous 

 substance of the brain, the second with the pia mater, and the 

 third with the dura mater. The interior of the eye contains 

 refractive media, viz., the lens and vitreous humour. In addition 

 there are the above-mentioned accessory structures: these, as well 

 as the retina, will be dealt with after a description of the eyes 

 of the various Classes of Vertebrates has been given. 



Before concluding this introductory section, it must be again 

 pointed out that the mode of development of the vertebrate eye 

 differs markedly from that of other sensory organs in not being 

 derived directly from the outer ectoderm, and in that the optic nerve 

 is not an ordinary peripheral nerve, but a conducting tract passing 

 between different parts of the central organ itself that is, between 

 the retina and brain. On the other hand, it must be borne in 

 mind that the sensitive elements are developed in that part of the 

 retinal layer which is originally continuous with the outer border 

 of the ectoderm that is, from the same layer as that which gives 

 rise to the other sensory organs and the central nervous system. 



Amphioxus. Paired eyes, comparable to those of the Craniata, 

 are wanting in the Acrania. 



In addition to an unpaired pigment-spot in the front Avail of the 

 cerebral vesicle, which is supposed to serve as a light-perceiving 

 organ, a scries of simple cup-like structures, somewhat resembling 

 the eyes of Flat Worms, are situated on either side of the spinal 

 cord (Fig. 204). These structures are arranged in groups, corre- 

 sponding to the myomeres, and gradually becoming less numerous in 

 passing backwards to the tail. Each of these bodies is described 

 as consisting of a single optic cell with a nerve fibre, and is partly 

 surrounded by a pigment cell. More dorsally, above the oral 

 region, somewhat similar, but pigmentless, bodies have been 

 observed. 



