732 THE CIRCULATORY SYSTEM 



connective tissue and smooth muscle develop from the adjacent mesenchyme 

 present in the area in which the capillary makes its appearance. 



a. Arteries 



The arteries are the system of blood vessels which convey the blood from 

 the heart to the systemic organs. Most arteries are composed of three coats 

 of tissue which come to surround the endothelium of the capillary, namely, 

 an inner tunica intima, a middle tunica media, and an outer tunica adventitia. 

 The tunica media is composed of smooth muscle fibers and elastic connective 

 tissue fibers, while the other two coatings are fabricated of connective tissue 

 fibers. 



In the large arteries in the immediate vicinity of the heart, the tunica media 

 is poorly muscularized but its elastic fibers are plentiful. However, in the 

 more distally placed arteries, the so-called distributing arteries which include 

 most of the arteries, the tunica media is supplied copiously with smooth 

 muscle fibers. 



b. Veins 



The veins are the vascular tubes which convey the blood from the systemic 

 organs back to the heart. The walls of the veins are more delicate than those 

 of the arteries, and the various tunics mentioned above are thinner, especially 

 the tunica media. The veins of the extremities form internal, pocket-shaped 

 valves which prevent the blood from moving backward. 



c. Capillaries 



The capillaries which form the ramifying bed of blood vessels between the 

 arteries and veins retain the primitive condition, and their walls are composed 

 of flattened endothelial cells. The size of the arteries and the thickness of the 

 arterial walls decrease as they approach the capillary bed, while those of the 

 veins increase as they leave the capillary area. 



4. Hematopoiesis (Hemopoiesis) 



a. Theories of Blood-cell Origin 



Hematopoiesis is the name given to the process which effects the forma- 

 tion of blood cells. Though it is agreed that blood cells generally arise from 

 mesenchymal cells, all students of the problem do not concur in the belief 

 that all arise from a specific type of mesenchymal cell. For example, in the 

 quotation given above from Stockard, '15, it is stated that one type of mesen- 

 chymal cell gives origin to the red blood cells, while leukocytes or white 

 blood cells arise from a slightly different type of mesenchymal cell. This may 

 be called the dualistic theory of hematopoiesis. The view held today by many 

 in this field of development is that all blood cells arise from fixed, undif- 



