STRUCTURE AND PROPERTIES OF THE VEINS 71 



with a large number of the smallest variety of the elastic fibres. This 

 layer contains capillary vessels (vasa vasorum). The circular fibres 

 are composed of elastic tissue, some of the fibres of the same variety as 

 is found in the longitudinal layer, some of medium size, and some in the 

 form of a " fenestrated membrane." In addition, there are inelastic 

 fibres interlacing in every direction and mingled with capillary blood- 

 vessels and non-striated muscular fibres. In the human subject, in the 

 veins of the central portion of the nervous system, the dura mater, the 

 pia mater, the bones, 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, mesenteric and axillary veins, there are both longitu- 

 dinal and circular fibres. In the smaller veins the fibres are circular. 

 In the smallest veins the middle coat is composed of fine fibres of con- 

 nective 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, 

 particularly 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 auricle (see Plate III, Fig. i). 



The venous sinuses and the veins that pass through bony tissue 

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

 the vessels being closely attached to the surrounding parts. As exam- 

 ples 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 more closely adherent to the surrounding tissues than the 

 arteries, particularly when they pass between layers of aponeurosis. 

 When a vein is cut across, its walls fall together, if not supported by 

 adhesions to surrounding tissues, so that its calibre is nearly or quite 

 obliterated. The elastic tissue, which gives to the larger arteries their 

 great thickness, is scanty in the veins, and the thin walls collapse when 

 not sustained by liquid in the interior of the vessels. 



Although with much thinner walls, the veins, as a rule, will resist a 

 greater pressure than the arteries. Wintringham (174) 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 



