162 



These panetes, which are much thinner than those of the arteries, are 

 contained, like theirs, in a sheath common to all the vessels. Three coats, 

 likewise, enter into their structure ; the middle or fibrous coat is not very 

 distinct, and consist merely of a few longitudinal reddish fibres, which can 

 be distinguished only in the larger veins, near the heart. In some of the 

 larger quadrupeds, as in the ox, these fibres form distinct fasciculi, and 

 their muscularity is much more manifest. 



The internal coat, which is more extensible that that of the arteries, 

 and equally thin, adheres more closely to the other coats. The cellular 

 coat, which connects it to the middle one, is less abundant, hence phos- 

 phate of lime is seldom deposited into it, as happens to the arteries which 

 frequently become ossified, as we advance in years. This internal coat is 

 merely a continuation of that which lines the cavities of the heart; and as 

 the origin of the inner coat of the arteries is the same, there exists a non- 

 interrupted continuity in the membrane which lines all the canals of the 

 circulation. The inner coat forms the only essential part of the venous 

 system; it alone constitutes the veins within the bones, the sinuses of the 

 dura mater, the hepatic veins, in a word, all the veins which are so firmly 

 attached externally to the neighbouring parts, that the blood flows along 

 them, as along inert tubes, their parietes being, almost completely inca- 

 pable of contracting. 



The veins, in their passage through muscles, are, like the arteries, 

 guarded by aponeurotic rings, than which is more remarkable than that 

 which belongs to the aperture in the diaphragm, which transmits the as- 

 cending cava from the abdomen into the thorax. This vessel is, therefore, 

 not compressed by the contraction of that muscle in inspiration. 



LXIV. As the inferior cava passes through the lower edge of the liver, 

 whether along a deep fissure, or in a real canal in the parenchymatous 

 substance of that viscus, the course of the blood must be impeded, when, 

 from congestion of the parenchyma, the vessel is, in some sort strangu- 

 lated. 



Obstruction of the liver, which is of such frequent occurrence, would 

 have been attended with fatal consequences, by preventing the return of 

 the blood from the inferior parts, along the ascending cava, if this great 

 venous trunk did not keep up, by means of the vena azygos, an open and 

 free communication with the descending or superior cava. The use of 

 this anastomosis of the two great veins is, evidently, to facilitate the pas- 

 sage of the blood from the one of these vessels into the other, when either, 

 especially the lower, does not readily evacuate its contents into the right 

 auricle. On this account, the vena azygos is capable of considerable 

 dilatation, and is entirely without valves. In the body of a man opened, 

 in my presence, and whose liver was twice as large as in health, I ob- 



speculated on the subject. It is, however, shown by Haller, that the vena cava, at 

 least, is muscular, and Verschuyr and other respectable physiologists, have detected 

 the same structure in the most minute veins. There is one act which xve think ought 

 alone to convince us of the contractility of these vessels, which is, that they always 

 adapt themselves to the quantity of blood they contain. 



Co-operating with the above cause is the action of the muscles, as may be illustrated 

 by the familiar example of venaesection. When in this case the blood issues languidly 

 from the orifice made in the vein, it is known, that nothing promotes its flow so effec- 

 tually as pressing something hard in the hand. This operates simply by bringing into 

 action the muscles of the fore-arm and humerus, thereby producing considerable com- 

 pression of the veins. Chapman. 



