230 GENERAL ANATOMY OF THE SANOUIFEROUS SYSTEM. 



pierce in convenient positions to discharge their contents in the 

 deeper seated veins. 



The trunks of the veins, in those places where no branches 

 go off, are generally cylindrical. There are, however, some 

 exceptions, in which these vessels are irregularly dilated, as 

 sometimes happens in the case of the internal jugular vein. It 

 is, however, not easy to determine from the appearance of 

 veins injected after death, respecting their situation during life, 

 as their coats are very yielding ; and it is very probable that 

 they are, therefore, preternaturally dilated by the injection. 



Veins, directly or indirectly, originate from the termination 

 of arteries : but they do not pulsate as the arteries do, because 

 the impulse given to the blood by the heart is very much 

 diminished in consequence of the great diminution of the size of 

 the vessels through which the blood has passed. 



In some cases, however, when blood flows from an opened 

 vein, the extent of its projection is alternately increased and 

 diminished, in quick succession, as if it were influenced by the 

 pulsation of the heart. 



The Coats of Veins differ considerably from those of 

 Arteries, for they are thinner, and so much less firm, that 

 veins, unlike arteries, collapse when they are empty. 



They consist of a dense elastic substance, the fibres of which 

 are much less distinct than those of arteries, but some of them 

 are to be seen in a longitudinal direction. These fibres can be 

 made to contract by local irritation ; for if a vein be laid bare 

 in a living animal, and then punctured, it will often contract so 

 as to diminish its diameter very considerably, although no blood 

 shall have escaped from the punctures. 



Next to the elastic substance is the internal coat, which is 

 smooth and polished. It is separated from the substance exterior 

 to it with difficulty, although it may be taken from it very easily 

 in the vena cava.* 



* The microscopical observers have found considerable analogy in the 

 structure of the arteries and veins, though their opinions vary in some respects. 

 The arteries are described by Henle as being composed of six different layers. 

 1. The innermost epithelial lining. 2. A striated or fenestrated coat. 3. A 

 granular fibrous layer. 4. The middle elastic or muscular coat, composed of 



