94 CIRCULATION OF THE BLOOD. 



In attempting to compare the quantity of blood normally circulating in the veins with 

 that contained in the arteries, we find such variations in the venous system at different 

 times and in different parts, both in the quantity of blood, rapidity of circulation, pressure, 

 etc., that a definite estimate is impossible. It would be unphilosophical to attempt even 

 an approximate comparison, as the variations in the venous circulation constitute one of 

 its greatest and most important physiological peculiarities, which must be fully appreciated 

 in order to form a just idea of the function of the veins. The arteries are always full, 

 and their tension is subject to comparatively slight variations. Following the blood into 

 the capillaries, we observe the immense modifications in the circulation with varying physi- 

 ological conditions of the parts, to which we have already referred. As would naturally be 

 expected, the condition of the veins varies with the changes in the capillaries, from which 

 the blood is taken. In addition to this, there are independent variations, as in the erectile 

 tissues, in the veins of the alimentary canal during absorption, in veins subject to press- 

 ure, etc. 



Following the veins in their course, it is observed that anastomoses with each other 

 form the rule, and not the exception as in the arteries. There are always a number of 

 channels by which the blood may be returned from a part; and, if one vessel be 

 obstructed from any cause, the current is simply diverted into another. The veins do 

 not present a true anastomosing plexus, such as exists in the capillary system, but simply 

 an arrangement by which the blood can readily find its way back to the heart, and by 

 which the vessels may accommodate themselves to the immense variations in the quan- 

 tity of their fluid contents. This, with the peculiar valvular arrangement which exists 

 in all but the veins of the cavities, provides against obstruction to the flow of blood 

 through, as well as from the capillaries, in which it seems essential to the proper nutri- 

 tion and function of parts that the quantity and course of the blood should be regulated 

 exclusively through the arterial system. Special allusion to the different venous anas- 

 tomoses belongs to descriptive anatomy. Physiologically, the communications between the 

 different veins are such that the blood can always find a way to the heart, and, once 

 fairly out of the capillaries, it cannot react and influence the circulation of fresh blood 

 in the tissues. 



Collected in this way from all parts of the body, the blood is returned to the right 

 auricle, from the head and upper extremities by the superior vena cava, from the trunk 

 and lower extremities, by the inferior vena cava, and from the substance of the heart, by 

 the coronary veins. 



Structure and Properties of the Veins. The structure of the veins is somewhat more 

 complex and difficult of study than that of the arteries. Their walls, which are always 

 much thinner than the walls of the arteries, may be divided into quite a number of 

 layers ; but, for convenience of physiological description, we shall regard them as pre- 

 senting three distinct 'coats. These have properties which are tolerably distinctive, 

 although not as much so as the three coats of the arteries. 



The internal coat of the veins is a continuation of the single coat of the capillaries 

 and of the internal coat of the arteries. It is a simple, homogeneous membrane, some- 

 what thinner than in the arteries, lined by a delicate layer of polygonal epithelium. 



The middle coat is divided by some into two layers ; an internal layer, which is com- 

 posed chiefly of longitudinal fibres, and an external layer, in which the fibres have a 

 circular direction. These two layers are intimately adherent and are quite closely 

 attached to the internal coat. The longitudinal fibres are composed of connective-tissue 

 fibres mingled with a large number of the smallest variety of the elastic fibres. This 

 layer contains a large number of capillary vessels (vasa vasorum). The circular fibres 

 are composed of elastic tissue, some of the fibres of the same variety as is found in the 

 longitudinal layer, some of medium size, and some in the form of the " fenestrated mem- 

 brane." In addition, there are inelastic fibres interlacing in every direction and mingled 



