THE LIVER. 539 



system ; the waste material being finally excreted from the kidneys, in the 

 form, chiefly, of urea. The undue retention of these matters in the blood, 

 whether by the failure in action of the liver, or of the kidneys, gives rise to 

 great depression, and in extreme cases, to insensibility and death. (4) To 

 excrete carbonic acid. (5) To form bile. As the vessels which supply the 

 portal vein absorb nutritive matters from the food, it follows that, when an 

 animal is too highly fed, its liver will be unduly taxed, wdth the result of more 

 or less serious derangement. As the liver requires a large amount of oxygen 

 for the purification of, and changes wTought in, the blood ; the want of air 

 and exercise is specially injiu-ious to that organ. The rate of breathing 

 directly affects the rapidity of the circulation of blood through the liver ; 

 hence, a state of idleness will tend to induce congestion of that gland. The 

 effect of a hot atmosphere not only diminii-hes the amount of air taken into 

 the lungs — for the warmer the air, the more rarefied will it be — but, also, 

 brings on destructive changes in the structure of the liver. The skin, which is 

 peculiarly active in the horse, helps the liver in removing impurities 

 from the blood ; hence, when the functions of the skin are checked by 

 the i^resence of a large proportion of moisture in the air, the powers of the 

 liver will be unduly sti-ained. This fact appears to be one of the chief reasons 

 that hot, damp climates are peculiarly unsuitable to horses. As exercise 

 quickens the entire circulation, its absence will, naturally, render that of the 

 livei torpid. 



The liver is composed of a large number of lobules, which are about the size 

 of millet-seeds, and in which bile is manufactured. This yellow fluid is carried 

 from the lobules by bile-tubes, which unite, and finally form a common duct, 

 which discharges the bile and also the pancreatic juice into the small intestine, 

 in order that they may mingle with the semi-prepared food (chyme) that has 

 just quitted the stomach. The bile-tubes are lined with mucous membrane, 

 which, in a state of health, constantly secretes mucus, to lubricate these pas- 

 sages. SuiTounding this membrane, there is a coat of involuntary muscular 

 fibre which urges forwards the bile and mucus l)y its contractions. 



Bile acts as a natural purgative. Hence, when it is absent, the bowels 

 become constij)ated, and the dung emits an oft'ensive odour and assumes a 

 clay colour on account of not being tinged by the colouring matter of the bile. 

 When the bile is regularly discharged, there are often coffee-coloured patches 

 found on the dung ; a fact which is owing to an altered condition of that 

 secretion. Bile assists the pancreatic juice in forming an emulsion with the 

 fat contained in the chyme. These two fluids being alkaline, a soap is formed, 

 in which the oily particles are split up into a very fine state of division, so 

 that the chyme (now called chyle) assumes a white appearance. The object 

 of the minute division of fat is to facilitate its absorption. The fact of the 

 mucous membrane of the intestines being, moistened with bile quickens the 

 absorption, through this coat, of fat contained in the food ; hence, when the 

 amount of the bile which is discharged into the intestine, is deficient in 

 quantity, the animal will, in all likelihood, lose condition. Bile also aids in the 

 absorption of albuminous matters. 



Bile is composed of bile acids and colouring matter, which, as we have seen, 

 is derived from the colouring matter of the red corpuscles of the blood. " The 

 source of the bile acids has not been determined " (Hamilton). 



When congestion of the liver occurs, its vessels become over-filled with 

 blood, and as, at the outset of every case of inflammation, the function of the 

 attacked organ is stimulated, an increased supply of bile is secreted. The 

 liver now swells considerably ; the bile tubes become blocked up, owing to the 

 inflamed state of their mucous lining, and to the presence of the over-distended 

 blood-vessels ; and the whole gland becomes gorged with bile, of which little 

 or none, as shown by the clay colour of the dung, is discharged. The bile, 

 thus obstructed, is in part absorbed by the blood, and taken into the general 

 circulation, so that the various tissues acquire the characteristic tinge of 

 jaundice. The bile is finally excreted by the kidneys, and to a small extent 

 by the skin. 



