CHAPTER XIV. 



THE CIRCULATION OF THE BLOOD (Continued). 

 THE VASCULAR APPARATUS: ITS STRUCTURE AND FUNCTIONS. 



The systemic vascular apparatus consists of a closed system of vessels 

 extending from the left ventricle to the right auricle, and includes the arteries, 

 capillaries, and veins. Though serving as a whole to transmit blood from 

 the one side of the heart to the other, each one of these three divisions has 

 separate but related functions, which are dependent partly on differences in 

 structure and physiologic properties, and partly on their relation to the heart 

 and its physiologic activities. 



The Structure, Properties and Functions of the Arteries. The 

 arteries serve to transmit the blood ejected from the heart to the capillaries; 

 that this may be accomplished they divide and subdivide and ultimately 

 penetrate each and every area of the body. Their repeated division is 

 attended by a diminution in size, a decrease in the thickness and a change in 

 the structure of their walls. 



A typical artery consists of three coats: an internal, the tunica intima; a 

 middle, the tunica media; an external, the tunica adventitia. 



The internal coat consists of a structureless elastic basement membrane, 

 on the inner surface of which rests a layer of elongated spindle-shaped en- 

 dothelial cells. The middle coat consists of several layers of circularly 

 arranged, non-striated muscle-fibres, between which are networks of elastic 

 fibers. The external coat consists of bundles of connective tissue of the 

 white fibrous and yellow elastic varieties. Between the external and middle 

 coats there is an additional elastic membrane. In the small arteries there is 

 but a single layer of muscle-fibers. In the large arteries the elastic tissue 

 is very abundant, exceeding largely in amount the muscle-tissue. It is also 

 more closely and compactly arranged. The external coat is well developed 

 in the large arteries (Fig. 148). 



In virtue of the presence in their walls of both elastic and contrac- 

 tile elements, the arteries possess the two properties of elasticity and 

 contractility. 



The elasticity is especially well developed in the large arteries, which are 

 capable, therefore, of both distention and elongation, and, when the distend- 

 ing force is withdrawn, of returning to their previous condition. The 

 elasticity permits of a wide variation in the amount of blood the arterial 

 system can hold between its minimum and maximum distention. Thus 

 the capacity of the aorta and carotid artery of the rabbit can be increased 

 four times and six times respectively by raising the intra-arterial pressure 

 from o to 200 mm. of mercury. The elasticity also converts the intermittent 

 movement of the blood imparted to it by the heart as it is ejected from the 

 ventricle, into a remittent movement in the arteries and finally into the con- 



3*9 



