250 DISEASES OF THE HORSE. 
media, is elastic, dense, and of a yellow color, consisting of nonstri- 
ated 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 adventitia, is 
composed mainly of fine and closely woven bundles of white con- 
nective tissue, which chiefly run diagonally or obliquely around the 
vessel. In this coat the nutrient vessels, the vasa vasorum, form a 
capillary network, from which a few penerate as far as the muscular 
coat. 
The veins differ from arteries in possessing thinner walls, less 
elastic and muscular tissue, and for the most part a stronger tunica 
adventitia. They collapse when cut across or when they are empty. 
The majority of veins are provided with valves; these are folds of 
the lining membrane, strengthened by fibrous tissue. They favor the 
course of the blood and prevent its reflux. The nerves which supply 
both the arteries and the veins come from the sympathetic system. 
The smaller arteries terminate in the system of minute vessels known: 
as the capillaries, which are interposed between the termination of 
the arteries and the commencement of the veins. Their average 
diameter is about one three-thousandth of an inch. 
DISEASES OF THE HEART AND BLOOD VESSELS. 
In considering diseases of the heart we meet with many difficulties, 
depending much upon the position which this organ occupies in the 
animal. The shoulders cover so much of the anterior portion of the 
chest, and often in very heavy-muscled horses the chest walls are so 
thick that a satisfactory examination of the heart is attended with 
difficulty. Diseases of the heart are not uncommon among horses; 
the heart and its membranes are frequently involved in diseases of 
the respiratory organs, diseases of the kidneys, rheumatism, influ- 
enza, etc. Some of the diseases of this organ are never suspected by 
the ordinary observer during life, and are so difficult to diagnose 
with any degree of certainty that we will have to confine ourselves 
to a general outline, giving attention to such symptoms as may serve 
to lead to a knowledge of their existence, with directions for treat- 
ment, care, etc. : 
Nervous affections often produce prominent heart symptoms by 
causing functional disturbance of that organ, which, if removed, will 
leave the heart restored to perfect vigor and normal action. Organic 
changes involving the heart or valves, however, usually grow worse 
and eventually prove fatal. Therefore it is necessary that we arrive 
at an appreciation of the true nature and causes so that we may be 
able to form a true estimate of the possibilities for recovery or en- 
couragement for medical treatment. 
