J10 ZOOLOGY SECT. 



of the retina the rods and cones, although not directly formed from 

 the external ectoderm, as in Invertebrates, are ultimately traceable 

 into the superficial layer of ectoderm, since they are developed from 

 the inner layer of the optic vesicle, which is a prolongation of the 

 inner layer of the brain, and the latter is continuous before the 

 closure of the medullary groove with the ectoderm covering the 

 general surface of the body. 



The eye-ball is moved by six muscles (Fig. 796). Four of these 

 arise from the inner wall of the orbit, and pass, diverging as they 

 go, to their insertion round the equator of the eye. One of them 

 is dorsal in position, and is called the superior rectus (s. r.), a second 

 ventral, the inferior rectus (in. r.), a, third anterior, the anterior 

 or internal rectus (i. r.), and a fourth posterior, the posterior or 

 external rectus (e. r.). The usual names (internal and external) of 

 the two last-named muscles originate from their position in Man, 

 where, owing to the eye looking forwards instead of outwards, its 



anterior surface becomes in- 

 ternal, its posterior surface ex- 

 ternal. The two remaining 

 muscles usually arise from the 

 anterior region of the orbit, 

 and are inserted respectively 



int the dorsal and ventral 

 surface of the eye-ball. They 



are the superior (s. o.) and 

 inferior oblique (i. o.) muscles. 



The median or pineal eye (Fig. 

 7^7) is formed, in certain 

 cases, from the distal end of 

 FIG. TOO. Muscles ami nerves of the eye of the parietal organ already men- 



a Skate (semi-diagrammatic). ///. oculo- 4-; n -n Pr I 1> >ia flip -fnrm n-f a 



motor nerve ; IV. trochlear ; VI. abducent. tionea. It nas tne 



e. r. posterior rectus ; i. o. inferior oblique ; rounded Capsule, the Outer Or 



in. r. inferior rectus; i. r. anterior rectus; \. r ,1 n e 



or. wall of orbit ; s. o. superior oblique ; s. r. anterior portion OI the Wall OI 



superior rectus. ft ^ L formed Q 



elongated cells, while its posterior portion has the character of a 

 retina (M, r). The latter has a layer of nerve-fibres on its outer 

 and one of rod-like visual elements (r.) on its inner surface : it 

 thus agrees with the usual types of Invertebrate retina, and not 

 with that of the paired eye. 



The organ of hearing, like that of sight, presents quite peculiar 

 features. It arises in the embryo as a paired invagination of the 

 ectoderm in the region of the hind-brain, a shallow depression being 

 formed which deepens and becomes flask-shaped, and finally, as 

 a rule, loses its connection with the external ectoderm, forming a 

 closed sac surrounded by mesoderm. At first simple, it soon 

 becomes divided by a constriction into dorsal and ventral compart- 

 ments. The dorsal compartment is differentiated into an irregular 



