28 GENERAL STRUCTURE OF THE BODY. 



or veins, the venae comites. Usually the arteries occupy 

 protected situations and are straight in their course. 

 Where a vessel has to accommodate itself to the move- 

 ments of a part, however, it may be curved, as in the case 

 of the facial artery which is curled on itself to allow for 

 movements of the jaw. They anastomose or communi- 

 cate freely with one another, thus promoting equality of 

 distribution and pressure and making good circulation 

 possible even after the obliteration of a large vessel. 



The veins have three coats like the arteries, but they 

 are not so thick and the muscular coat is not so highly 

 developed, so that the walls collapse when cut and have 

 no elasticity. There are constrictions on the surface 

 of many of the veins due to the presence of valves. 

 These valves are formed of semi-lunar folds of the lining 

 membrane and are arranged in pairs. They serve to 

 prevent the blood, whose circulation in the veins is 

 sluggish, from flowing back. 



There are two sets of veins, the superficial and the 

 deep, which communicate with each other. In fact, 

 all the veins, large and small, anastomose very freely, 

 especially in the skull and neck, where obstruction 

 would result in serious trouble, throughout the spinal 

 cord, and in the abdomen and pelvis. The deep veins 

 accompany the arteries in their sheath, while the 

 superficial ones have thicker walls and run between the 

 layers of the superficial fascia under the skin, terminating 

 in the deep veins. In the skull the venous channels 

 take the form of sinuses, formed by a separating of the 

 layers of the dura mater, with an endothelial lining that 

 is continuous with that of the veins. 



The capillaries are intermediate between the arteries 

 and the veins, the final division of the arteries and the 

 first source of the veins. They are tiny vessels with 

 but a single coat, continuous with the innermost coat 

 of both arteries and veins and consisting practically of 

 one layer of cells with a small amount of connective tis- 

 sue between. They spread in a great network through- 



