90 CIRCULATION OF THE BLOOD IN THE VESSELS. 



are no muscular fibres in the middle coat. In the larger veins, such as the 

 abdominal vena cava, the iliac, crural, popliteal, mesenteric and axillary veins, 

 there are both longitudinal and circular fibres. In the smaller veins, the 

 fibres are circular. In the smallest veins, the middle coat is composed of 

 fine fibres of connective tissue with a very few muscular fibres. 



The external coat of the veins is composed of ordinary fibrous tissue, like 

 that of the corresponding coat of the arteries. In the largest veins, particu- 

 larly those of the abdominal cavity, this coat contains a layer of longitudinal, 

 non-striated 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 pulsa- 

 tion of the veins in the immediate vicinity of the heart is very marked. In 

 nearly all veins, the external coat is several times thicker than the internal 

 coat. This is most marked in the larger veins, in which the middle coat, 

 particularly the layer of muscular fibres, is very slightly developed. 



The venous sinuses and the veins which pass through bony tissue have 

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

 whole being closely attached to the surrounding parts. As examples, may be 

 mentioned the sinuses of the dura mater and the veins of the large bones of 

 the skull. In the first instance, 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 canals formed 

 by a layer of compact bony tissue. The veins are much more closely adher- 

 ent to the surrounding tissues than the arteries, particularly when they pass 

 between layers of aponeurosis. 



The peculiarities in the anatomy of the veins indicate considerable dif- 

 ferences 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 

 tissues, so that its caliber is nearly or quite obliterated. The 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 the vessels. 



Although with much thinner and apparently weaker walls, the veins, as a 

 rule, will resist a greater pressure than the arteries. Wintringham (1740) 

 showed that the inferior vena cava of a sheep, just above the opening of 

 the renal veins, was ruptured by a pressure of one hundred and seventy-six 

 pounds (79'8 kilos.), while the aorta, at a corresponding point, yielded to a 

 pressure of one hundred and fifty-eight pounds (71-7 kilos). The strength of 

 the portal vein was even greater, supporting a pressure of nearly five atmos- 

 pheres, bearing a relation to the vena cava of six to five ; yet these vessels had 

 hardly one-fifth the thickness of the arteries. In the lower extremities in the 

 human subject, the veins are much thicker and stronger than in other 

 situations, a provision against the increased pressure to which they are habit- 

 ually subjected in the upright posture. Wintringham noticed a singular 

 exception to the general rule just given. In the vessels of the glands and of 

 the spleen, the strength of the arteries was much greater than that of the 



