ANIMAL BIOLOGY. 



[Part I. 



layer of nerve fibres, through the intermediate layers, and thence 

 by the optic nerve to the brain. Thus the light passes outwards 

 through the thickness of the retina mechanically as through a 

 transparent medium, and then passes backwards through the 

 retina organically as a wave of molecular change through the 

 nerve endings. The granular layers owe their dotted appearance 

 to the (stained) nuclei in the specially modified cells. 



11. The Stomach and Intestine. A cross-section of a frog's 

 stomach examined under a low power (Fig. 28, i.) shows a num- 

 ber of inwardly projecting folds, and if it be stained there 



FIG. 28. STOMACH AND INTESTINE. 



i. Stomach of frog. ii. A portion of the same under higher power. 1. 

 Peritoneum. 2. Longitudinal muscular layer. 3. Concentric muscular layer. 

 4. Submucosa. 5. Muscularis mucosse. 6. Mucosa containing peptic glands, 

 iii. Peptic gland (dog) under still higher power, iv. Villus (dog) injected to 

 show blood-vessels, v. Villus (rabbit) stained to show cells. From sections. 



will be an internal lighter and an external darker zone. Under 

 a higher power (28, ii.) six tolerably distinct layers are seen : 

 (1.) Externally the peritoneum, often lost from the section; 

 (2.) external muscular -layer with fibres running longitudinally ; 

 (3.) internal muscular layer with fibres running concentrically ; 



