CHAPTER VI. 



ON THE VEINS. 



THE veins are the vessels which return the blood to the auricles 

 of the heart, after it has been circulated by the arteries through the 

 various tissues of the body. They are much thinner in structure 

 than the arteries, so that when emptied of their blood they become 

 flattened and collapsed. The veiris of the systemic circulation 

 convey the dark-coloured and impure or venous blood from the 

 capillary system to the right auricle of the heart, and they are 

 found after death to be more or less distended with that fluid. The 

 veins of the pulmonary circulation resemble the arteries of the 

 systemic circulation in containing during life the pure or arterial 

 blood, which they transmit from the capillaries of the lungs to the 

 left auricle. 



The veins commence by minute radicles in the capillaries which 

 are every where distributed through the textures of the body, and 

 converge to constitute larger and larger branches, till they termi- 

 nate in the large trunks which convey the venous blood directly to 

 the heart. In diameter they are much larger than the arteries, and 

 like those vessels their combined areas would constitute an imagi- 

 nary cone, whereof the apex is placed at the heart, and the base at 

 the surface of the body. It follows from this arrangement, that the 

 blood in returning to the heart is passing from a larger into a smaller 

 channel, and therefore increases in rapidity during its course. 



Veins admit of a threefold division, into superficial, deep, and 

 sinuses. 



The Superficial veins return the blood from the integument and 

 superficial structures, and take their course between the layers of 

 the superficial fascia ; they then pierce the deep fascia in the most 

 convenient and protected situations, and terminate in the deep 

 veins. They are unaccompanied by arteries, and are the vessels 

 usually selected for venesection. 



The Deep veins are situated among the deeper structures of the 

 body and generally in relation with the arteries ; in the limbs they 

 are enclosed in the same sheath with those vessels, and they return 

 the venous blood from the capillaries of the deep tissues. In com- 

 pany with all the smaller, and also with the secondary arteries, as 

 the brachial, radial, and ulnar in the upper, and the tibial and pero- 

 neal in the lower extremity, there are two veins, placed one on each 



