428 THE HORSE. 
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 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), ard the ch yle is 
by it carried into the veins through an opening at the junction of the left 
vena cava anterior, with the axillary 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 feces. 
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 becomes 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 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 consti- 
tutes a compound one, which, if the groups of which it is composed are 
loosely bound together like grapes, as in the salivary glands, is called con- 
glomerate ; while, if they are united into a solid mass, such as the 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 saliva, bile, &c., which are commonly known as secretions. 
These are all removed 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 being injurious to its welfare, and therefore to be discarded, 
The term secretion is sometimes confined to the former, while the latter 
action receives the distinguishing term excretion: but as in many cases 
the fluid which is removed as being injurious to the system is also used 
for beneficial purposes, the distinction is not capable of being strictly 
maintained. The nature of the process is essentially the same in all cases, 
being carried out by the development of simple cells, each possessing its 
own independent vitality. These cells select certain ingredients from the 
blood, and then set them free by the rupture of their walls; and being 
situated on the free surface of the lining membrane of the gland, which is 
continuous with the mucous membrane or skin, the secreted fluid gradually 
reaches the one or the other. It is impossible, at present, to ascertain the 
precise means by which each gland is made up of cells having special 
powers of selection; but that the fact is so is capable of demonstration. 
Thus, the cells of the liver select the elements of bile; those of the 
