STRUCTURE AND PROPERTIES OF THE VEINS. 95 



with capillary blood-vessels, and the unstriped or involuntary muscular fibres. In the 

 human subject, in the veins of the dura and pia mater, the bones, and the retina, the 

 vena cava descendens, the thoracic portion of the vena cava ascendens, the external and 

 internal jugulars, and the subclavian veins, there are no muscular fibres in the middle coat. 

 In the larger veins, such as the abdominal vena cava, the iliac, crural, popliteal, mesen- 

 teric, and axillary veins, the fibres are both circular and transverse. In the smaller 

 veins, the fibres are circular. 



The external coat of the veins is composed of white fibrous tissue, like the cor- 

 responding coat of the arteries. In the largest veins, particularly those of the abdominal 

 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 immediate 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 being closely attached to the surrounding parts. As examples of this, 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 mem- 

 brane is adherent to canals formed by a layer of compact bony tissue. The veins are 

 much more closely adherent to the surrounding tissues than the arteries, particularly 

 when they pass between layers of aponeurosis. 



The above peculiarities in the anatomy of the veins indicate 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 tissues, 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 the vessels. Whenever the veins remain open after 

 saction, it is on account of their attachment to surrounding tissues and is not due to 

 the rigidity of the vessels themselves. 



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

 resist a greater pressure than the arteries. Observations on the relative strength of 

 the arteries and veins were made by Hales, but the most extended experiments on the 

 subject were made by Clifton Wintringham, in 1740. The latter observer ascertained 

 that the inferior vena cava of a sheep, just above the opening of the renal veins, was rup- 

 tured by a pressure of one hundred and seventy-six pounds, while the aorta, at a corre- 

 sponding point, yielded to a pressure of one hundred and fifty-eight pounds. 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 habitually subjected in the upright posture. Win- 

 tringham noticed one 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 veins. The splenic vein gave way under a pressure of little more than one atmos- 

 phere, while the artery supported a pressure of more than six atmospheres. 



A little reflection on the influences to which the venous and arterial circulation are 

 subject will enable us to understand the physiological importance of the great difference 

 in the strength of the two varieties of vessels. It is true that in the arterial system thf 

 constant pressure is greater than in the veins; but it is nearly the same throughout the ar 



