STRUCTURE OF THE HORSE. 105 



three distinct coats — an internal, middle, and external. The 

 internal coat is formed of an epithelial layer, a sub-epithelial 

 layer, and an elastic layer. The middle coat is formed of muscle, 

 with a slight admixture of elastic tissue. The external coat 

 consists of fine, connective tissue, with a various amount of 

 elastic tissue arranged longitudinally. The external coat is very 

 tough, while the middle and internal are elastic and brittle. On 

 ligaturing an artery, the internal and middle coats give way, 

 while the external one is left unbroken. The heart contracts 

 and propels the blood from the left ventricle through the aorta, 

 the great artery of the whole body. This, after proceeding 

 about two inches, divides into two large branches. The smaller 

 branch is extended, by a multitude of sub-divisions, to every part 

 of the head and fore extremity, the larger one, in a similar 

 manner, throughout the body and hind extremities. In the smaller 

 arteries anastomoses are frequent, forming a net-work, which 

 pervades every tissue of the body. This is a point of great im- 

 portance, since the circulation can be thus carried on after the 

 main artery of the region has been obliterated; and it is proved 

 that under such circumstances the smaller arteries of the part 

 increase in size. As the blood nears the extremities, the arte- 

 ries grow smaller, being divided and sub-divided to supply the 

 various tissues of the body. The blood ultimately reaches the 

 capillaries, which are interposed between the termination of the 

 arteries and the commencement of the veins, forming plexuses, 

 which A'ary much in arrangement. The capillary vessels are 

 very small, their diameter being about 1.3000 of an inch. The 

 blood flows through the capillaries with less velocity than in 

 arteries or veins. The flow is constant, not intermittent, as in 

 the larger arteries. AVhile ])assing through the capillaries the 

 blood unloads its nutritive material and takes on effete or waste 

 material, conveying it back to the heart through the veins. 



The veins carry the blood from the capillaries to the heart. 

 They ramify through the body like the arteries, but arc more 

 numerous, anastomose more freely, and are of greater capacity 



