36 HIGH SCHOOL ZOOLOGY. 



is formed by the single layer of tall columnar cells- — the pigmentary 

 epithelium of the retina, while the other, the retina proper, is formeil of 

 several layers, of which that toward the cavity is the layer of the rods 

 and cones, while that towards the vitreous humor is the layer of optic 

 nerve fibres. Between the two are various layers of nerve-cells, sup- 

 ported by other elements whicli are not nervous in their nature. 

 {Fig. 14). The rods and c(mcs are the neuro-epithelial cells of the re- 

 tina, and the original space between them and the pigmentary epithe- 

 ium is obliterated by the close contact of the two layers. The lens on 

 the other hand is shown to be developed from the epidermis, and the 

 fibres of which it is composed are really altered epidermal cells. 



45. The Ear. — In man tlie ear consists of three parts, the 

 external ear, the middle ear or drum-ckvity, and the internal ear 

 or labyrinth : only the latter exists in the catfish. The two 

 former in man are concerned with the concent lation of sound- 

 waves on the latter ; how then in the absence of these, do 

 sound-waves reach the labyrinth in the catfish? In some fishes 

 a rudimentary gill-cleft between the hyoid arch and the jaws, 

 appears to be the channel through which the vibrations reach 

 the internal ear, but no such gill-cleft exists in the cattish, so it 

 is probable that they are transmitted through the bones of the 

 head, and above all through the comparatively loose ones, 

 which are suspended to the ear c:'i)sule, by the hyomandibular 

 (^5 21). Another possible channel will be referred to after- 

 wards. In all animals the labyrinth is the essential part of the 

 organ of hearing, as it is in it, that the auditory nerve termin- 

 ates. In most forms the labyrinth is enclosed in a complete 

 cartilaginous or bony capsule, (forming in the latter case the 

 proo^ic, e\)iotic bones, &c.,) and only perforated by small aper- 

 tures towards the outside and towards the cavity of the skull, 

 but in the group of fishes to which the catfish belongs, the side 

 of the capsule towards the brain is very deficient, and conse- 

 quently the greater part of the labyrinth can be seen by opening 

 the cranial cavity. 



