52 STRUCTURE AND ECONOMY OF THE HORSE, 



three coats ; the outer, the strongest and thickest, gives to the 

 vessel the remarkable elasticity which it possesses ; the middle 

 coat is the fibrous, which seems to be a modification of muscular 

 power, and enables the arteries to contract on their contents; the 

 third coat is the serous, which lubricates the interior of the 

 vessel, and facilitates the passage of the blood. Thus, to these 

 several coats, but particularly to the two former ones, do the 

 arteries owe the remarkable property they possess, of contract- 

 ing when distended with blood, and almost immediately after- 

 wards expanding to receive a fresh supply ; and which, assisted 

 by the action of the heart, constitutes the pulse, and may be felt 

 in every part of the body, where an artery is sufficiently near 

 the surface of the skin to be perceptible. 



The arteries, however, do not all possess an equal thickness 

 and power ; for instance, the pulmonary artery, tliough quite as 

 large as the aorta, is neither so thick nor so strong ; and the 

 reason is, that the same power is not required to send the blood 

 over the smaller circuit of the lungs as over the larger one of 

 the whole system ; and for the same reason, the right side of the 

 heart is weaker than the left. 



The arteries, as thev divide and subdivide in their course, 

 become weaker in their coats in proportion to the diminution of 

 their size, till at length they terminate in the minute branches 

 called the capillary vessels, which do not possess any pulsating 

 power, and many of which do not contain red blood. Dimi- 

 nutive, however, as these branches may be, yet it is by them 

 that the most important offices are performed; by them the 

 different parts of the body are nourished, whether bone, flesh, 

 nerve, or skin ; by them the various fluids are secreted, how- 

 ever different in their appearance they may be; by them the 

 most gliastly wounds are healed, and often in a remarkably 

 short space of time ; and all these various offices are performed 

 not only by the same class of vessels, but by the same fluid, the 

 blood. 



Having accomplished these important purposes, the capillary 

 arteries terminate in equally minute vessels, called the capillary 

 veins ; and so abundant are these diminutive vessels, that the 

 finest point of the finest needle cannot be plunged into the body 

 without penetrating some of them. By the time the blood 

 reaches the veins, it becomes dark and impure, and loaded with 

 carbon ; the office of the veins, therefore, is to return it to the 

 heart to be again purified. The circulation, however, becomes 

 much slower, as it is further removed from the impulsive power 

 of the heart ; and the veins, which are supposed to contain two 

 thirds of the whole blood circulating in the system, are conse- 

 quently much more numerous than the arteries, They do not, 

 however, possess the same strength in their coats as the arteries. 



