64 CIRCULATION OF THE BLOOD. 



CHAPTER III. 



CIRCULATION OF THE BLOOD IN THE VESSELS. 



Physiological anatomy of the arteries Course of blood in the arteries Locomotion of the arteries and production 

 of the pulse Pressure of blood in the arteries Pressure in different parts of the arterial system Depressor 

 nerve Influence of respiration on the arterial pressure Eapidity of the current of blood in the arteries Eapid- 

 ity in different parts of the arterial system Circulation of the blood in the capillaries Physiological anatomy 

 of the capillaries Capacity of the capillary system Course of blood in the capillaries Eolations of the capil- 

 lary circulation to respiration Causes of the capillary circulation Influence of temperature on the capillary cir- 

 culationInfluence of direct irritation on the capillary circulation Circulation of the blood in the veins Physio- 

 logical anatomy of the veins Course of the blood in the veins Pressure of blood in the veins Eapidity of the 

 venous circulation Causes of the venous circulation Air in the veins Function of the valves Conditions 

 which impede the venous circulation Eegurgitant venous pulse Circulation in the cranial cavity Circulation 

 in erectile tissues Derivative circulation Pulmonary circulation Eapidity of the circulation Phenomena in 

 the circulatory system after death. 



IN man and in all animals possessed of a double heart, each contraction of this organ 

 forces a charge of blood from the right ventricle into the pulmonary artery, and from 

 the left ventricle into the aorta. We have seen how the valves which guard the orifices 

 of these vessels effectually prevent regurgitation during the intervals of contraction. 

 There is, therefore, but one direction in which the blood can flow in obedience to this 

 intermittent force ; and the fact that, even in the smallest arteries, there is an accelera- 

 tion in the current coincident with each contraction of the heart, which disappears when 

 the action of the heart is arrested, shows that the ventricular systole is the prime cause 

 of the arterial -circulation. But this part of the physiology of the circulation is not so 

 simple as we might at first be led to suppose. The arteries have the important function 

 of supplying nutritive matter to all the tissues, of furnishing to the glands materials out 

 of which the secretions are formed, and, in sliort, are the vessels of supply to every part 

 of the organism. The supply of blood regulates, to a considerable extent, the processes of 

 nutrition and has an important bearing on the general and special functions ; and the 

 various physiological processes necessarily demand considerable modifications in the 

 quantity of arterial blood which is furnished to parts at different times. For example, 

 during secretion, the glands require several times as much blood as in the intervals of 

 their action. The force of the heart, we have seen, varies but little within the limits of 

 health ; and the conditions necessary to the proper distribution of blood in the economy 

 are regulated almost exclusively by the arterial system. These vessels are not inert 

 tubes, but are endowed with elasticity, by which the circulation is considerably facili- 

 tated, and with contractility, by which the supply to any part may be modified, inde- 

 pendently of the action of the heart. Sudden flushes or pallor of the countenance are 

 examples of the facility with which this may be effected. It is evident, therefore, that 

 the properties of the coats of the arteries are of great physiological importance. We 

 shall then commence the study of this division of the circulatory system with a consid- 

 eration of its physiological anatomy. 



Physiological Anatomy of the Arteries. 



The vessels which carry the venous blood to the lungs are branches of a great trunk 

 which takes its origin from the right ventricle. They do not differ in structure from the 

 vessels which carry the blood to the general system, except in the fact that their coats 

 are somewhat thinner and more distensible. The aorta, branches and ramifications of 

 which supply all parts of the body, is given off from the left ventricle. Just at its ori- 

 gin, behind the semilunar valves, the aorta has three sacculated pouches, called the si- 

 nuses of Valsalva. Beyond this point the vessels are cylindrical. As we recede from 

 the heart, the arteries branch, divide, and subdivide, until they are reduced to micro- 



