54 STRUCTURE AND ECONOMY OE THE HORSE. 



is greatly assisted by the action of the valves in supporting the 

 column of blood. The blood, thus supported and propelled by 

 the arteries, assisted by atmospheric pressure, must go somewhere, 

 as the valves prevent return : it goes, therefore, where a vacancy 

 is afforded, and that is in the right auricle of the heart, which 

 has just propelled its contents into the ventricle. To these 

 several forces may be added a power of suction the heart possesses 

 whenever the chest is enlarged in respiration. 



The manner in which the chyle is mixed up with the blood, 

 so that its colour quickly disappears, is worthy of particular 

 notice. It is owing, indeed, to the great agitation the blood 

 receives, and to the irregularity of the heart's internal surface. 

 When the auricles contract, their contents are in a great mea- 

 sure discharged into the ventricles, but a portion is thrown back 

 into the veins, which constitutes Avhat is called the venous pulse, 

 and may sometimes be seen in the jugular veins. In like 

 manner, when the venti'icles contract, a portion of their con- 

 tents is thrown back into the auricles, at any rate that part of 

 it situated behind the valves. By these means an agitation is 

 produced, which effectually mixes these different fluids to- 

 gether. 



It has been ascertained that the veins possess a power of 

 absorption in common with a numerous class of vessels called 

 the absorbents, or lympliatics. These vessels are very minute, 

 and are distributed throughout the whole body ; they generally 

 accompany the veins, and, like them, are furnished with valves ; 

 sometimes, as in farcy, they become inflamed, and greatly en- 

 larged. In the course of these absorbent vessels there are a 

 great number of glands, in which they appear to terminate, and 

 others to begin, and thus their contents are passed onwards into 

 the circulation. These glands are subject to disease occasion- 

 ally, as in strangles ; and it is the hardness and swelling of these 

 glands under the jaw that gives its name to the disease called 

 glanders. By rubbing various medicines on the skin, they are 

 taken up by the absorbent vessels, and carried into the circula- 

 tion ; and any interruption in the action of these vessels pro- 

 duces dropsical swellings. — Ed.] 



CHAP. XII. 



THE BLOOD ITS NATURE AND COMPOSITION. 



[The Blood is justly regarded as the most important fluid in 

 the animal machine : it stimulates the heart to contract, secretes 

 and nourishes the various organs, and supplies the body with 



