280 THE HORSE. 



PHYSIOLOGY OF DIGESTION. 



BEFORE PROCEEDING TO EXAMINE into the anatomy of the ab- 

 dominal 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 expul- 

 sion 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 compounds 

 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 mace- 

 ration ; 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 duo- 

 denum for that purpose. Here it is further elaborated, and re- 

 ceives the bile and pancreatic juice, which are poured out through 

 their ducts opening on the internal surface of this intestine. The 

 nutricious parts of the food are now gradually converted into chyle; 

 and as it passes into the jejunum and ilium, it is there absorbed 

 by the lymphatics (here called lacteals), whose mouths open upon 

 the villi thickly lining this part of the canal. These unite into 

 one duct (the thoracic^, and the chyle is by it carried into the 

 veins through an opening at the junction of the left vena cava an- 

 terior, with the allary vein. From the small intestines, the food, 

 minus its nutritive portions, is passed on into the large intestines, 

 and finally reaches the rectum and anus, in the form known as 

 faeces. The peculiar offices performed by the bile and pancreatic 

 fluid will be described under the sections treating of each of those 

 organs. 



THE ABSORPTION OF FLUID from the interior of the alimentary 

 canal is effected in two different modes first, by the lacteals, which 

 take up the chyle through their open mouths ; secondly, by the 

 veins, which absorb it through their walls by the process known 

 as endosmose. In the former case the chyle is at once carried to 

 the heart; but in the latter, it passes through the liver, and be- 

 comes purified and chemically altered in that organ. The lacteals 

 pass through the mesenteric glands, which lie between the layers 

 of the mesentery. 



STRUCTURE OF GLANDS AND PHYSIOLOGY OF 

 SECRETION. 



A GLAND may be defined to be an organ whose office it is to 

 separate from the blood some peculiar substance, which is poured 



