172 CIRCULATORY SYSTEM. 



. In some cases the veins follow precisely the course of the ar- 

 teries, one for one, as in the greater number of the viscera of 

 the abdomen, where they have common points of entering and 

 departure. Sometimes two arteries discharge into one vein, as 

 in the penis, the clitoris, and the umbilical chord; sometimes 

 they pursue a course entirely different from the arteries, as in 

 the pia mater. For the most part they are less tortuous than 

 the arteries. 



The veins, when injected, assume a cylindrical shape, yet 

 they differ materially from the arteries, in having much thinner 

 coats, and in being so pliable that they collapse by their own 

 weight. In the lower extremities, however, near the feet and 

 upon them, as the veins sustain the pressure of a long column 

 . of blood, they have additional thickness and strength, so as to 

 approximate them more to the arterial structure. This provision 

 will be found occurring in most places where they have much 

 duty to perform. 



"They are similar to the arteries in their structure, being 

 composed of an elastic and muscular substance ; the elasticity 

 preserves them in some degree in a middle state, although not 

 so perfectly as it does in the arteries. The muscular power 

 adapts the veins to the various circumstances, which require 

 the area to be within the middle state, and assists the blood in 

 its motion toward-s the heart."* 



The External Coat is thinner and not so strong as that of the 

 arteries; in other respects, the resemblance is sufficiently close 

 not to require any particular comment. 



The Middle Coat, near the entrance of the .larger veins into 

 the heart, is distinctly muscular.t It is formed of soft extensible 

 fibres, many of which, when the vein is held up to the light, ap- 

 pear longitudinal, while the most internal are circular: there are 

 difficulties, however, in the separation of these fibres, which pre- 

 vent their course from being accurately ascertained. Bichat 

 and Meckel assert, that the whole of them are longitudinal, and 

 that there are none circular. 



This coat, in the human subject, is much thicker in the system 



* Hunter, loc. cit. t B^clard, loc. cit,. 



