288 CHORD ATE ANATOMY 



Because of the strength and elasticity of their walls, arteries usually do not 

 collapse after death. 



Arteries have their own circulatory and nervous supply. The capil- 

 laries of the walls are the vasa vasorum. The nerves are branches of the 

 autonomic system, and are of two sorts, vasoconstrictor nerves which 

 stimulate the contraction of the blood-vessels and thus check the flow of 

 blood, and vasodilator nerves which act as inhibitors and thus permit the 

 dilation of the blood-vessels. 



Puhnonary Circulation in Man. The pulmonary artery carries 

 impure or "venous" blood from the right ventricle to the lungs. Near the 

 heart it divides into right and left branches which connect with the 

 corresponding lungs. At the point of separation of right and left pul- 

 monary arteries the ligamentum arteriosum connects them with the aorta. 

 From the lungs, blood is returned to the left atrium by pulmonary 

 veins which, unlike other veins, convey aerated blood. While there are 

 usually four pulmonary veins, occasionally there are five and the blood 

 from the middle right lobe enters the atrium independently. Within 

 the lung lobes the pulmonary veins parallel the arteries. 



Systemic Circulation in Man. The systemic circulation in man 

 differs in no essentials from that in other mammals. In a brief text such 

 as this, space does not permit a detailed description of the arteries and 

 veins in the human body. But a comparison of the human arteries and 

 veins shown in Figures 258 and 259 with those of the cat will convince 

 the student of their fundamental similarity. For this purpose the 

 figures are probably more useful than detailed description would be. 



The Lymphatic System in Man. The process of nutrition of the 

 tissues involves constant loss of fluid or ' ' leakage " from the capillaries. It 

 is generally thought to be due to three factors, blood pressure, osmosis, and 

 diffusion; but active secretion by the endothelial cells may also be involved. 

 The fluid which passes from the capillaries forms lymph. Lymph 

 is essentially blood plasma from which the corpuscles, except a few 

 leucocytes, have been removed. The lymph lost from the blood capil- 

 laries is taken up by the lymphatic capillaries which, like the blood capil- 

 laries, are microscopic vessels whose walls consist of a single layer of 

 endotheHum. Unlike blood capillaries, however, the lymphatics vary 

 greatly in size, and in the translucent mesentery are visible to the naked 

 eye when distended with the milky lymph absorbed from the intestine 

 after a meal. (Fig. 251) 



The lymphatic system includes capillaries, collecting vessels, and 

 lymph nodes. Like blood capillaries, lymph capillaries exchange dissolved 

 substances with the surrounding tissues. They form compUcated plexuses, 

 of which few parts of the body are devoid. 



