524 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



contraction of the ventricles. At each systole of the ventricles the heart 

 completely empties itself, and, consequently, throws into the blood- 

 vessels the amount of blood capable of being contained in its cavity, 

 while at the same time an equal quantity of blood enters the heart from 

 the veins. This injection of new amounts of blood into the arterial 

 system, as a consequence, occurs intermittently, as may be readily 

 recognized by opening the artery of an animal, when it will be found 

 that the blood will issue in spurts, each spurt corresponding to a con- 

 traction of the ventricles. It will, however, be remarked that there is 

 also an escape of blood during the pauses of the contractions of the 

 heart, and that the farther from the heart a blood-vessel be opened the 

 less will be the apparent effect of ventricular contraction in increas- 

 ing the velocity with which the blood flows from the divided vessel. In 

 other words, the blood within the blood-vessels is subjected to a con- 

 siderable tension, derived from the elasticity of the vascular walls, and 

 this tension itself serves partly to assist in the onward movement of the 

 blood. When a vessel divides, except in rare instances, the sum of the 

 calibre of the branches is, as a rule, greater than that of the parent stem. 

 In nearly all cases, however, the capacit}- of the branches is consider- 

 ably greater than the original vessels before division, even though the 

 sum of the diameters of the branches be but little greater than that of 

 the parent stem. The arterial system may thus be regarded as a cone 

 whose apex joins the left ventricle, and whose base is represented by the 

 capillary system. The venous system, on the other hand, may be repre- 

 sented by an inverted cone, whose base is formed by the capillaries, and 

 whose apex is in communication with the right auricle. 



In the arteries the conditions of the movements of the blood are 

 largely governed by the physical characteristics of the walls of the blood- 

 vessels. The arteries consist of three coats — an inner serous coat or 

 endothelium, the middle elastic and muscular coat, and an outer fibrous 

 coat. It is to the middle coat that the physical characters of the circu- 

 lation are largely due. The proportion of muscular fibre to elastic tissue 

 varies considerably in different parts of the arterial system. In the 

 large arteries directly in the neighborhood of the heart the middle coat 

 is composed almost solely of yellow elastic tissue, while the muscular 

 fibres are present in small amount. As the capillary system is ap- 

 proached, or, in other words, as the arteries become smaller and smaller 

 bj T repeated subdivisions, the elastic coat diminishes in amount, while the 

 muscular coat increases. The relative proportions of these two elements 

 of the middle coat are, therefore, inversely as the diameter of the vessel. 



The action of these two elements is to a certain extent antagonistic, 

 although they both combined serve to assist in the onward movement of 

 the blood. 



