478 THE ANATOMY OF THE HOil«E 



and mixed with the .sali\;i, which acts as a kind uf J'ermcnl iu cuuvertiiig 

 the starchy matters, which form so large a proportion of the horse's food, 

 into sugar, and, Avith 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 maceration ; but this organ being 

 small in the horse, it cannot remain tlicre 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 

 receives the bile and pancreatic juice, which are poured out through their 

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

 parts of the food ai'e now gradually converted into chi/Ie ; and as it passes 

 into the jejunum and ilium, it is there absorbed by the lymphatics (here 

 called lacteah), whose mouths open upon the villi thickl}^ 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 

 ca\'a anterior with the axillary vein. From the small intestines, the food, 

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

 finally I'eaches the rectum and auus, in the form known as fteces. 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 txuiD 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 becomes purifietl and chemically altcre<i 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 out through an 

 excretory duct, whose internal surface is continuous with the mucous 

 membrane, or skin. A simple gland is, in fact, nothing more than a pouch 

 of mucous membrane ; and a collection of these pouches constitutes a 

 compound one, which, if the groups of which it is composed an; loosely 

 bound together like grapes, as in the salivary glands, is called coiiglomcrate ; 

 while if they are united into a solid mass, such as (he liver, the term 

 conglobate is applied. 



By secretion is understood the process of separation of various matters 

 from the blood ; the term being also applied to the products of the process, 

 such as sa.liva, bile, etc., which are commonly known as secretions. These 

 are all i-emoved from the blood for one of two purposes — first, in order to 

 be employed for some ulterior object in the various processes going on in 

 the body, either for its own preservation, or that of others ; or, secondly, as 



