OP THE VEINS. 299 



447. The veins, like all the vessels, are surrounded by the 

 cellular tissue of the parts in which they are situated, which 

 forms a sheath, loose around the trunks, but more intimately 

 united with the twigs. The sheath of the vena porta is re- 

 markable in the liver, where it is known under the name of 

 capsule of Glisson. 



The outer membrane, properly so called, is thinner and less 

 condensed than that of the arteries, to which it bears a great re- 

 semblance. The inner membrane is formed of fibres more ex- 

 tensible and softer than those of the arteries. These fibres appear 

 nearly all longitudinal, when the membrane is examined and 

 held between the eye and the light; some of the most inter- 

 nal fibres seem annular; but when we wish to separate the 

 fibres of this membrane, the same difficulty is experienced in 

 all directions. In the human species this membrane is much 

 thicker in the system of the inferior vena cava than in the 

 other; generally it is also thicker in the superficial than in 

 the deep seated veins; thus the internal saphenavein has very 

 thick parietes at the lower part of the leg. Near their en- 

 trance into the heart, the veins have distinct muscular fibres. 

 The inner membrane, which is thin and transparent, differs 

 from that of the arteries by its extensibility and its resistance 

 to rupture, and by its filamentous texture, which becomes evi- 

 dent when it is distended and torn. The large veins of the 

 cranium or sinuses, the veins of the bones and some others, 

 are almost entirely constituted by the inner membrane, and 

 are besides, as it were, scooped out in the substance of the dura 

 mater, the bones, &c. 



The parietes of the veins are provided with small blood 

 vessels and nervous filaments, which may be followed for a 

 certain extent. 



448. The parietes of the veins are whitish, semi-transpa- 

 rent, thinner than those of the arteries; generally their thick- 

 ness augments absolutely from the roots towards the trunks, 

 and diminishes, when compared with the diameter, by follow- 

 ing the same course. Their density is of 115 or 110; the 

 firmness of their walls is much less than that of the arteries, 

 for this reason they collapse when empty, with the exception 



