DISEASES OF THE HORSE. 227 



nerve is cut, the heart-beats increase rapidl}-, and in fact the organ 

 works without control. When the nerve is undulj" irritated the hold- 

 back, or inhibitory force, is increased, and the heart slows up in the 

 same measure. The left cavities of the heart, the pulmonar}" veins, 

 and the aorta, or S3^stemic artery, contain red or florid blood, fit to cir- 

 culate through the body. The right cavities of the heart, with the 

 vena? cava^, or sj^stemic veins, and pulmonary artery, contain dark 

 blood, which must be transmitted through the kings for renovation. 



The arteries, commencing in two great trunks, the aorta and the 

 pulmonary artery, undergo division, as in the branching of a tree. 

 Their branches mostl}^ come off at acute angles, and are commonly of 

 uniform diameter in each case, but successive!}^ diminish after and in 

 consequence of division, and in this manner gradually merge into the 

 capillary system of blood vessels. As a general rule, the combined 

 area of the branches is greater than that of the vessels from which 

 they emanate, and hence the collective capacity of the arterial system 

 is greatest at the capillary vessels. The same rule applies to the veins. 

 The effect of the division of the arteries is to make the blood move more 

 slowly along their branches to the capillar}^ vessels, and the effect of 

 the union of the branches of the veins is to accelerate the speed of the 

 blood as it returns from the capillar}' vessels to the venous trunks. 



In the smaller vessels a frequent running together, or anastomosis^ 

 occurs. This admits of a free communication between the currents of 

 blood, and must tend to promote equabilit}' of distribution and of pres- 

 sure, and to obviate the effects of local interruption. The arteries are 

 highly elastic, being extensile and retractile both in length and breadth. 

 During life thej" are also contractile, being provided with muscular 

 tissue. When cut across the}' present, although empty, an open orifice; 

 the veins, on the other hand, collapse. 



In most parts of the body the arteries are inclosed in a sheath formed 

 of connective tissue, but are connected so loosely that, when the vessel 

 is cut across, its ends readily retract some distance within the sheath. 

 Independently of this sheath, arteries are usually described as being 

 formed of three coats, named, from their relative positions, external, 

 middle, and internal. This applies to their structure so far as it is 

 discernible by the naked eye. The internal, serous, or tunica intiina^ 

 is the thinnest, and is continuous with the lining membrane of the heart. 

 It is made up of two layers— an inner, consisting of a layer of epithelial 

 scales, and an outer, transparent, whitish, highly elastic, and perforated. 

 The middle coat, tunica media, is elastic, dense, and of a yellow color, 

 consisting of nonstriated muscular and elastic fibers, thickest in the 

 largest arteries and becoming thinner in the smaller. In the smallest 

 vessels it is almost entirely muscular. The external coat, tunica adven- 

 titia, is composed mainly of fine and closely woven bundles of white 

 connective tissue, Avhich chiefly run diagonally or obliquely round the 



