PHYSIOLOGY OF DIGESTION. 427 
sets, one circular, and the other collected in separate longitudinal bands. 
By the consecutive action of these fibres (called peristaltic), the food is 
driven onwards from one end to the other of the alimentary canal. 
THE MUCOUS MEMBRANE, Which lines the whole length of the alimentary 
canal, from the mouth to the anus, is continuous with the skin at these 
two orifices—with the mucous membrane lining the air-passages of the 
lungs at the entrance to the larynx—with that investing the nasal 
passages and cavities at the antero-superior part of the pharynx—and, 
lastly, with the internal ear through the eustachian tubes which open 
into the back of the pharynx. It is also reflected into the ducts of the 
salivary glands, which open into the mouth, and into those of the liver 
and pancreas, so that it has very extensive communications with these 
several organs. Like the skin, this membrane has a base composed of 
primary membrane, called the coriwm, on which are scattered the glands 
that secrete the gastric juice, imbedded in loose areolar tissue. In the 
intestines we shall find it extensively supplied with absorbents, which 
open upon its velvety pile or villi, and the whole protected by epithelium, 
which serves an important part in the production of the mucus every- 
where found upon its surface when in a healthy state. Inthe cesophagus 
it is thick, and disposed in longitudinal folds, allowing of lateral disten- 
tion. In the stomach it exists in coarse folds or rug, and in the intes- 
tines it is gathered into sharp folds, chiefly manifested in the duodenum. 
It is extensively supplied with blood throughout its whole surface, but 
especially where it lines the stomach and small intestines, and it is also 
liberally furnished with nerves, chiefly derived from the great sympathetic 
system. 
THE ABDOMINAL VISCERA ARE SUPPLIED WITH BLOOD by tranches from 
the aorta, passing between the folds of the peritoneum to reach their 
destination, excepting in the cases of the kidneys and pancreas, which 
have no such folds. The same folds also include the veins returning the 
blood to form the vena ports (see page 417), and also the lymphatics and 
chyliferous absorbents, to be hereafter described. 
THE NERVES are chiefly derived from the great sympathetic system ; 
but branches from the cerebro-spinal system are also distributed to the 
contents of the abdomen, and especially to the stomach, by means of the 
pheumogastric nerve. 
PHYSIOLOGY OF DIGESTION. 
BEFORE PROCEEDING TO EXAMINE into the anatomy of the abdominal 
organs, it may be well to investigate the nature of the processes which 
are carried out by them. To do this, the food must be traced from its 
prehension by the lips and teeth to its expulsion from the anus. Thus, 
commencing with the mouth, we find it there ground into a coarse pulp, 
and mixed with the saliva, which acts as a kind of ferment in converting 
the starchy matters, which form so large a proportion of the horse’s food, 
into sugar, and, with the aid of the gastric juice, into the proteine com- 
pounds necessary for the formation of flesh.» Perfect mastication and 
insalivation are therefore highly important processes to healthy digestion. 
When it reaches the stomach, the food undergoes still further changes by 
the agency of the gastric juice and of maceration ; but this organ being 
small in the horse, it cannot remain there long enough to be converted into 
perfect chyme (the result of the first process of digestion), but is passed on 
into the duodenum for that purpose. Here it is further elaborated, and 
