270 THE HORSE. 



is to allow of tilt free passa;j:e onwards of tli< blood, but not of its 

 return by regurgitation. If they become diseased, the action of 

 the heart is impeded, and the circulation of the blood is more or 

 less seriously interfered with. So, also, if the muscular fibres, of 

 which the walls of the auricles and in much thicker layers of the 

 ventricles, are composed, become weak by want of proper exercise, 

 or from the deposit of fat in their interspaces, a corresponding de- 

 gree of mischief is effected in the passage of the blood. The force 

 with which the left ventricle contracts may be estimated from the 

 fact, that if a pipe is inserted in the carotid artery of a horse, and 

 held perpendicularly, the blood will rise in it to a height of ten 

 feet; and the rapidity of his circulation is such, that a saline sub- 

 stance will pass from the veins of the upper part of the body to 

 those of the lower in little more than twenty seconds. Now, aa 

 this transmission can only take place through the current that re- 

 turns to the heart, and passes thence through the lungs and back 

 again, afterwards being forced into the lower vessels through the 

 aorta, it follows that every particle of this fluid passes completely 

 through the whole circulation in the above short period of time. 



THE VEINS. 



The veins generally correspond with the arteries, the blood of 

 which they return to the heart. Thus, there is a large vein which 

 conveys all the blood from the anterior half of the body supplied 

 by the anterior aorta, and this is called vena cava anterior. In a 

 similar manner the ixMerior vena cava is made up of veins which 

 accompany the several arteries that are found throughout the body, 

 with one remarkable exception connected with the secretion of bile. 



Although, in general, the veins and arteries correspond in 

 their ramifications, yet there is a large class of superficial veins 

 which are not accompanied by any of the latter vessels. In horses 

 which for many generations have been accustomed to fast work, 

 these superficial veins are strongly developed, and are particularly 

 plain in the Arab and his descendants. As a consequence of this, 

 and of the fact that many of the arteries are accompanied by two 

 veins, the whole number of veins is much greater than that of the 

 arteries, and the internal area of the former may be considered to 

 be nearly double tliat of the latter. In their walls the veins are 

 much thinner than the arteries, though, like them, they have three 

 coats, the serous and cellular being very similar in structure, but 

 the fibrous is very much thinner and devoid of muscular fibres. 

 A feature peculiar to the veins is the existence of valves, which 

 are sometimes single, at others double, and occasionally arranged 

 ii' threes and fours around the interior of the large veins. They 

 vary in numbers, and are altogether absent in the pulmonary veins 

 in the venae cavje, and the vena portae. 



