22 ANATOMICAL CONSIDERATIONS. 



moves against gravity. When the head is erect, and the venous cur- 

 rent, flowing toward the heart, is of course downward or is favored by 

 gravity, then the valves do not act ; but the passage of the blood forces 

 the duplicatures of membrane to remain close against the sides of the 

 tube. 



The jugulars conduct the dark-colored blood from the brain ; and as 

 that important organ cannot endure the smallest pressure, some special 

 provision was imperative to carry away the fluid, and also to anticipate 

 the possibility of its return to oppress the sensorium. When the horse 

 is grazing, the head is lower than the heart, and it naturally occupies 

 that position for the greater portion of the twenty-four hours. During 

 all that time the venous current must mount against the influence of 

 gravitation ; and to aid the reader in properly understanding the means by 

 which this is efi'ected, his attention is invited to the following diagrams. 



A SKETCH, mSPLATINQ THE ACTION OP THE JOQULAR VALVES WHEN THE HEAD IS LOWERED TO FEED OFF THE 



GROUND. 



The elevated crest, therefore, presents a clear channel to the vital 

 current. For that reason, the violent action or the most rapid pace of 

 the animal never produces congestion of its brain. The racer may sink 

 from exhaustion, but does not perish from apoplexy. The head, when 

 depressed, however, shows the same canal divided by numerous inter- 

 secting marks. Such lines are intended to represent the venous valves, 

 which assist the blood in its upward journey, and render impossible the 

 slightest pressure upon the sensorium. The first thing which strikes 

 the reader, upon beholding the arrangement depicted above, is the vast, 

 number of valves; and this causes him to inquire, where was the 



