OUTLINE OF THE STRUCTURE OF THE HORSE. 43 



exceeds the serum. Every one can readily find opportuni- 

 ties of marking the difference between these constituents 

 of the blood. It is only necessary to let rest for a few 

 minutes a quantity of the latter, fresh drawn, when a coag- 

 ulation will take place, by which the clot and serum will be 

 separated. 



The circulatory process in animal life suggests the com- 

 parison of a pond or lake, first fed by a few considerable 

 streams, which have been formed by other and smaller ones, 

 and these, in turn, by a multitude of little rills, originating 

 in drops of water oozing almost imperceptibly from the 

 earth; then drained by other channels, which divide and 

 subdivide into innumerable rivulets and trickling streams, 

 until, at last, all become absorbed and lost beneath the sur- 

 face. Yet from the reservoir, hidden in the bowels of the 

 earth, the water finds its way to the surface, where it again 

 oozes out in drops, which accumulate in streamlets and 

 rivers, to feed the lake as before. Again the outlets spread 

 out upon the other side, and ramify, until they are swallowed 

 up beneath the surface. Thus the round continues indefi- 

 nitely. 



In a similar manner the functions of circulation are car- 

 ried forward. The heart is the reservoir; the veins, the 

 feeding streams; the arteries, the streams that flow away 

 upon the other side. 



The circulatory system of the horse, like that in the hu- 

 man body, consists of the arteries, veins, and capillaries. 

 The arteries are the vessels which convey the red, oxygen- 

 ized blood from the heart to every part of the body. The 

 capillaries are the net- work of minute vessels which ramify 

 through every organ and part, and, though generally spoken 

 of as constituting a distinct system of blood-vessels, should 

 properly be regarded as simply the termination of the arte- 

 ries and the commencement of the veins— the connecting 

 links between the arterial and ven9us systems. The veins 

 are the ducts, through which the blood, now become of a 

 dark color, returns to the heart. 



