PHYSIOLOGICAL ANATOMY OP THE VEINS. 89 



as in the erectile tissues, in the veins of the alimentary canal during absorp- 

 tion, in veins subject to pressure, 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 

 "is 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 frequent variations in the quantity 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 essen- 

 tial to the proper nutrition and action of parts that the quantity and course 

 of the blood should be regulated exclusively through the arterial system. 



Collected by the veins 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 

 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 number of layers ; 

 but for convenience of physiological description, they may be regarded as 

 presenting three distinct coats. These have properties which are somewhat 

 distinctive for each, although not as much so as those of 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, homogene- 

 ous membrane, somewhat thinner than in the arteries, lined by a delicate 

 layer of polygonal endothelium, the cells of which are shorter and broader 

 than the endothelial cells of the arteries. 



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

 ternal layer, which is 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 mingled with a 

 large number of the smallest variety of the elastic fibres. This layer con- 

 tains 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 membrane." In addition, there are inelastic fibres inter- 

 lacing in every direction and mingled with capillary blood-vessels, and non- 

 striated muscular fibres. In the human subject, in the veins of the central 

 portion of the nervous system, the dura mater, the pia mater, the bones, the 

 retina, the vena cava descendens, the thoracic portion of the vena cava 

 ascendens, the external and internal jugulars and the subclavian veins, there 



