XIII 



PHYLUM CHORDATA 



107 



are mesodermal : the lens and external epithelium of the cornea 

 are derived from the ectoderm of the head : the retina and optic 

 nerve are developed from a hollow pouch of the brain, and are 

 therefore, in their ultimate origin, ectodermal. The sensory cells 

 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, which 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. 742). 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 (in- 

 ternal and external) of the 

 two last-named muscles origin- 

 ate from their position in Man, 

 where, owing to the eye look- 

 ing forwards instead of out- 

 wards, its anterior surface be- 

 comes internal, its posterior 

 surface external. The two re- 

 maining muscles usually arise 

 from the anterior (in Man 

 inner) corner of the orbit, and 



are inserted respectively into 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. 743), is formed, in certain cases, 

 from the distal end of the epiphysial diverticulum already men- 

 tioned. It has the form of a rounded capsule, the outer or 

 anterior portion of the wall of which is a lens (7.) formed of 

 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 sur- 

 face : 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 in vagi nation of the 



VI 



FIG. 74-2. Muscles of the eye of a Skate and 

 their nerves (semi-diagrammatic). ///. oculo- 

 motor nerve ; IV, trochlear ; VI, abducent. 

 e. r. external rectus ; in. o. inferior oblique ; 

 'm. r. inferior rectus; i. r. internal rectus; 

 or. wall of orbit ; s. o. superior oblique ; s. /. 

 superior rectus. 



