70 CIRCULATION OF THE BLOOD 



in different parts of the body. In some situations the capacity of the 

 veins and arteries is about equal ; while in others, as in the pia mater, 

 the veins contain, when fully distended, six times as much blood as the 

 arteries. 



In attempting to compare the quantity of blood normally circulating 

 in the veins with that contained in the arteries, such variations are found 

 at different times and in different parts, both in the quantity of blood, 

 rapidity of circulation, pressure etc., that a definite estimate is impos- 

 sible. It would be unprofitable to attempt even an approximate com- 

 parison, as the variations in the venous circulation constitute one of 

 its most important physiological peculiarities, which must be fully 

 appreciated in order to form a just idea of the uses of the veins. 

 The arteries are always full, and their tension is subject to com- 

 paratively slight changes. Following the blood into the capillaries, 

 important modifications in the circulation are observed under varying 

 physiological conditions of the parts. As would naturally be expected, 

 the condition of the veins varies with the changes in the capillaries 

 from which the blood is received. 



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 is always a number of channels by which the blood may be re- 

 turned from a part; and if one vessel is obstructed from any cause, 

 the current is 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 may readily find its way back to 

 the heart, and by which the vessels can accommodate themselves to 

 variations in the quantity of their contents. 



Structure and Properties of the Veins. The structure of the veins 

 is more complex than that of the arteries. Their walls, which are always 

 much thinner than the walls of the arteries, may be divided into a num- 

 ber of layers ; but for convenience of physiological description, they 

 may be regarded as presenting three coats. 



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 in the form 

 of a fenestrated membrane, somewhat thinner than in the arteries, lined 

 with a delicate layer of polygonal endothelium, the cells of which are 

 shorter and broader than the endothelial cells lining the arteries. 



The middle coat is divided by some anatomists into two layers ; an 

 internal layer, composed 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 



