THE DIGESTIVE SYSTEM 93 



muscle contracts this tension is reduced so that the elastic reaction 

 of the lens causes its curvature to increase. The size of the aperture, 

 the pupil, through which light reaches the lens is regulated by a 

 deeply pigmented, muscular diaphragm, the iris. 



By the structures described in the previous paragraph, light 

 rays are brought to a focus upon the retina, the layer of nervous 

 tissue lining the large cavity which occupies most of the eye. This 

 cavity is filled with a gelatinous vitreous body. 



The outermost layer of the retina (i.e. that nearest the sclera) 

 is formed by a single row of deeply pigmented epithelial cells, which 

 prevent the light from passing further. Into these project the rods 

 and cones,* the actual receptive nerve endings. Internal to the 

 rods and cones are several layers containing the cell bodies to which 

 these endings belong and many other nerve cells of various kinds, 

 together with supporting elements. Thus, in order to reach the 

 rods and cones, the light must pass through these internal layers, 

 which are almost perfectly transparent when alive. The most 

 internal layer of nerve cells gives rise to the fibres of the optic 

 nerve, and. these converge over the inner surface to one point, 

 where they turn abruptly outward, penetrating the whole thickness 

 of the wall of the eye and proceeding to the brain. 



In dissection, the nervous portion of the retina separates readily 

 from the outer pigmented epithelial layer, but the latter is firmly 

 adherent to a surrounding coat, the chorioid membrane, which is 

 also deeply pigmented, contains very numerous blood vessels, and 

 is loosely attached externally to the inner surface of the sclera. 

 Towards the exposed side of the eye, the chorioid membrane passes 

 over into the ciliary body and the iris, these three together consti- 

 tuting the vascular tunic of the eye. 



The Digestive System 

 The digestive system comprises as its chief portions the diges- 

 tive tube or alimentary canal and the digestive glands. The 

 digestive tube is divisible into several parts, which, with the 

 exception of the caecum and its vermiform process, are arranged in 

 a linear series. The digestive glands comprise the oral glands, the 



*The rabbit has very few cones. Correspondingly, it is reported to be 

 colour-blind, the cones being the colour-sensitive receptors. 



