STRUCTURE OF THE VEINS. 305 



cavity, this coat contains a layer of longitudinal unstriped 

 muscular fibres. In the veins near the heart, are found a few 

 striated fibres, which are continued on to the veins from the 

 auricles. In some of the inferior animals, as the turtle, these 

 fibres are quite thick, and pulsation of the veins in the imme- 

 diate vicinity of the heart is very marked. 



In nearly all veins, the external coat is several ' times 

 thicker than the internal. This is most marked in the larger 

 veins, in which the middle coat, particularly the layer of 

 muscular fibres, is very slightly developed. 



In what are called the venous sinuses, and in the veins 

 which pass through bony tissue, we have only the internal 

 coat, to which are superadded a few longitudinal fibres, the 

 whole closely attached to the surrounding parts. As exam- 

 ples of this, may be mentioned the sinuses of the dura mater, 

 and the veins of the large bones of the skull. In the first in- 

 stance, there is little more than the internal coat of the vein 

 firmly attached to the surrounding layers of the dura mater. 

 In the second instance, the same thin membrane is adherent 

 to bony canals formed by a layer of compact tissue. The 

 veins are much more closely adherent to the surrounding tis- 

 sues than the arteries, particularly when they pass between 

 layers of aponeurosis. This fact has been pointed out by 

 Berard l as very general, and is one to which he attaches 

 considerable physiological importance. He considers that 

 this arrangement serves to keep the veins open and give 

 them additional strength. 



The above peculiarities in the anatomy of the veins indi- 

 cate considerable differences in their properties, as compared 

 with the arteries. When a vein is cut across, its walls fall 

 together, if not supported by adhesions to surrounding tis- 

 sues, so that its caliber is nearly or quite obliterated. The 

 yellow elastic tissue, which gives to the larger arteries their 

 great thickness, is very scanty in the veins, and the thin 

 walls collapse when not sustained by liquid in the interior of 



2 Op. cit., tome iv., p. 9. 

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