DIGESTION 



235 



orifice. It possesses three coats, of which the external is a thin layer of 

 peritoneum, the free surface of which is extremely smooth and polished, 

 and is kept constantly moist to permit of movement against the adjoining 

 viscera with the least possible friction; folds of this layer, named omenta, 

 keep the stomach in position and attach it to the liver and spleen. The 

 middle coat or layer L muscular, the outermost fibres of which run longi- 

 tudinally and the innermost circularly; between the two is a net-work of 

 nerve fibres. The muscular coat enables the stomach to contract, and by 

 so doing to accommodate itself to the 

 quantity of its contents, and it also 

 propels the food into the intestine. 

 This it accomplishes by an undulating 

 or worm-like motion called peristalsis. 

 The peculiar valve-like arrangement of 

 the muscular tissue near the oesophageal 

 opening at which the food enters, as 

 well as the position of this aperture, 

 explain the difficulty that horses ex- 

 perience in vomiting. The third and 

 internal coat is the mucous coat, which 

 presents a striking difference in its ap- 

 pearance in its right and left portions. 

 That of the left half (a, fig. 82) presents 

 a white aspect, and is covered with 

 flattened epithelial cells, which form a 

 thick membrane lining the stomach, 

 beneath which are the numerous small 

 prominences or papillae of the subjacent 

 mucous tissue. The right half, on the contrary, which commences abruptly 

 by a sinuous line where the left terminates, is soft, of pink colour, and 

 vascular, and presents the openings of many thousands of glands, named 

 peptic glands, which secrete the gastric juice. An example (fig. 83) of 

 such a gland is here shown. The duct, it will be observed, is wide at the 

 upper end, where it opens into the general cavity of the stomach, but soon 

 divides into two or more, which terminate below in blind extremities. 



The gastric juice is a clear fluid of acid reaction, which is secreted in 

 large quantity when food is taken into the stomach. It contains very little 

 solid matter, the proportion of water being nearly 99 '5 per cent, but there 

 are in it two constituents which exert a powerful influence on the process of 

 digestion — one a ferment named pepsine and the other hydrochloric acid. 



The Small intestine. — The small intestine commences at the 





Lacteal vessels. ;1 Muscular 

 ■oat. 4 Serous coat. 



