STRUCTURE OF VEINS. 335 



olood, which they transmit from the capillaries of the lungs to the left 

 auricle. 



The veins commence by minute radicles in the capillaries which are 

 everywhere distributed through the textures of the body, an,d converge to 

 constitute larger and larger branches, till they terminate in the main trunks 

 which convey the venous blood directly to the heart. In diameter they 

 are larger than the arteries, and, like those vessels, their combined arece 

 would constitute an imaginary cone, whereof the apex is placed at the 

 heart, and the base at the surface of the body. It follows from this ar- 

 rangement, that the blood in returning to the heart is passing from a larger 

 into a smaller channel, and therefore that it 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 super- 

 ficial structures, and take their course between the layers of the superficial 

 fascia; they then pierce the deep fascia in the most convenient arid pro- 

 tected situations, and terminate in the deep veins. They are unaccompa- 

 nied 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 company with all the smaller, 

 and also with the secondary arteries, as the brachial, radial, and ulnar in 

 the upper, and the tibia! and peroneal in the lower extremity, there are 

 two veins, placed one on each side of the artery, and named vena comites. 

 The larger arteries, as the axillary, subclavian, carotid, popliteal, femoral, 

 &c., are accompanied by a single venous trunk. Sinuses differ from veins 

 in their structure ; and also in their mode of distribution, being confined 

 to especial organs and situated within their substance. The principal 

 venous sinuses are those of the dura mater, the diploe, the cancellous 

 structure of bones, and the uterus. 



The communications between veins are even more frequent than those 

 of arteries, and they take place between the larger as well as among the 

 smaller vessels ; the vena? comites communicate with each other very fre- 

 quently in their course, by means of short transverse branches which pass 

 across from one to the other. These communications are strikingly ex- 

 hibited in the frequent inosculations of the spinal veins, and in the various 

 venous plexuses, as the spermatic plexus, vesical plexus, &c. The office 

 of these inosculations is very apparent, as tending to obviate the obstruc- 

 tions to which the veins are particularly liable from the thinness of their 

 coats, and from their inability to overcome much impediment by the force 

 of their current. 



Veins, like arteries, are composed of three coats, external or areolo- 

 fibrous, middle or fibrous, and internal or serous. The external coat is 

 firm and strong, and resembles that of arteries. The middle coat consists 

 of two layers, an outer layer of contractile fibrous tissue disposed in a cir- 

 cular direction around the vessel, and an inner layer of organic muscular 

 fibres arranged longitudinally. This latter resembles the inner layer of 

 the middle coat of arteries, but is somewhat thicker ; it is not unfrequently 

 hypertrophied. The internal coat, as in arteries, consists of a striated or 

 fenestrated layer, and a layer of epithelium ; it is continuous with the in- 

 ternal coat of arteries through the medium of the lining membrane of t).^ 



