338 ZOOLOGY FOR MEDICAL STUDENTS chap. 



the optical image thrown on it by the lens into terms of nerve-impulses, 

 is of extraordinarily complicated structure and the details even in our 

 present imperfect knowledge of them are too complex to be gone into 

 in an elementary text-book. The general structure is as follows. There 

 is a layer of sensory^ — in this case visual — cells of the slender columnar 

 form so common in sensory cells. Instead of an ordinary sensory hair 

 each of these carries at its free end a rod — a column of highly specialized 

 cuticular material, alternate transverse layers of which present a dimmer 

 and a more transparent appearance. It is apparently the substance of 

 this rod, composed of peculiarly modified cytoplasm, that possesses the 

 extraordinary property of converting waves of light into living impulses. 

 The layer of visual cells is underlaid by a thick mass of ganglion-cells, 

 of various different types and linked up together in highly complicated 

 fashion, and then eventually, on the surface of this furthest from the 

 rods, there emerge the nerve-fibres which collect together to form the 

 optic nerve. The complicated mass" of ganglion-cells and visual cells is 

 supported by special straight cells, with much frayed out cell -body 

 composed of stiff cytoplasm, which traverse the retina at intervals 

 in a direction perpendicular to its surface. Closely abutting on the 

 face of the retina bearing the rods is a layer of cells (pigment layer), 

 polygonal in shape as seen in surface view, their cytoplasm laden with 

 black melanin pigment. These cells during exposure to bright light 

 push out numerous pigment-laden pseudopodia into the spaces between 

 the rods while in faint light these are withdrawn. 



One at first sight extraordinary feature of the percipient part of the 

 vertebrate retina is that it is reversed in position from what we should 

 expect — the visual cells with their rods pointing not towards the lens, 

 i.e. in the direction from which the stimulus comes, but away from it 

 (Fig. 141, r). It follows that the light rays in order to reach the rods 

 have first to traverse all the rest of the retina, and in accordance with 

 this the retina of the vertebrate is absolutely transparent. 



The lens and retina are enclosed within the eyeball — a protective 

 capsule composed of very tough connective tissue. The greater part of 

 this has a characteristic white opaque appearance and is known as the 

 sclerotic (Fig. 141, s). It contains in the Dogfish a layer of cartilage. 

 On its outer side a circular portion of the capsule loses its opacity and 

 becomes absolutely clear and transparent — the cornea (Fig. 141, C). 

 The space between the sclerotic and the pigment layer of the retina is 

 occupied by a spongy layer known as the choroid (Fig. 141, ch), very 

 rich in blood-vessels and laden with melanin pigment. The outer edge 

 of the cup-like arrangement formed by the choroid with its lining of 



