60 CIRCULATION OF THE BLOOD IN THE VESSELS. 



CHAPTER III. 



CIRCULATION OF TEE BLOOD IX THE VESSELS. 



Physiological anatomy of the arteries Courso 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 sys- 

 tem Depressor nerve Influence of respiration on the arterial pressure Rapidity of the current of blood 

 in the arteries Rapidity in different parts of the arterial system Circulation of the blood in the capil- 

 lariesPhysiological anatomy of the capillaries Pressure of blood in the capillaries Relations of the 

 capillary circulation to respiration Causes of the capillary circulation Influence of temperature on the 

 capillary circulation Influence of direct irritation on the capillary circulation Circulation of the blood 

 in the veins Physiological anatomy of the veins Course of the blood in the veins Pressure of blood in 

 the veins Rapidity of the venous circulation Causes of the venous circulation Air in the veins Uses 

 of the valves Conditions which impede the venous circulation Regurgitant venous pulse Circulation 

 in the cranial cavity Circulation in erectile tissues Derivative circulation Pulmonary circulation Cir- 

 culation in the walls of the heart Passage of the blood-corpuscles through the walls of the vessels (dia- 

 pedesis) Rapidity of the circulation Phenomena in the circulatory system after death. 



Ix man and in all animals possessed of a double heart, each cardiac con- 

 traction forces a charge of blood from the right ventricle into the pulmo- 

 nary artery, and from the left ventricle into the aorta ; and 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 acceleration 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 cause of the arterial cir- 

 culation. The arteries have the important office of supplying nutritive mat- 

 ters to all the tissues and furnishing to the glands materials out of which the 

 secretions are formed, and, in short, 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 spe- 

 cial functions; and the various physiological processes necessarily demand 

 considerable modifications in the quantity of arterial blood which is furnished 

 to parts at different times. The force of the heart, however, varies but little 

 within the limits of health ; and the conditions necessary to the proper distri- 

 bution of blood in the economy are regulated almost exclusively by the arte- 

 rial system. These vessels are endowed with elasticity, by which the circula- 

 tion is considerably facilitated, and with contractility, by which the supply to 

 any part may be modified, independently 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. 



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 sys- 

 tem, except in the fact that their coats are somewhat thinner and more dis- 

 tensible. The aorta, branches and ramifications of which supply all parts of 



