1 68 HISTOLOGY 



through them in single file. Their walls are composed of elongated, very 

 flat cells, with irregularly wavy polygonal outlines which are clearly 

 demonstrated in silver nitrate preparations (Fig. 162). Between the cells, 

 the red and white corpuscles frequently make their way out of the vessel. 

 There are no pre-formed openings for this purpose, and the endothelial 

 cells come together after the corpuscles have passed out. Certain en- 

 dothelial cells are phagocytic, devouring objects which float in the blood; 

 some of them may become detached and enter the circulation. More- 

 over endothelial cells are contractile, and may be stimulated to activity 



by the sympathetic fibers in the delicate 

 perivascular plexus which is shown in 

 methylene blue preparations. Some of the 

 fibers end in contact with the cells and pre- 

 sumably control the caliber of the vessel; 

 other fibers may be afferent and receive a 

 stimulus when the vessel expands and 



FIG. 162. CAPILLARY FROM THE TAIL , , , , , , , , . , , 



OF A TADPOLE. SILVER NITRATE Stretches the pleXUS. The bulging of CndO- 

 PREPARATION. (After Koelliker) ,-,., i , ,1 i ir 



thehal nuclei into the lumen of vessels, fre- 

 quently seen in preserved specimens, is probably due to post-mortem 

 contraction; in life the lining is presumably smooth. 



Although capillaries vary in diameter (4.5-12 /*), those in a giv-en 

 territory are quite uniform, both as to caliber of individual vessels and 

 the size and pattern of the meshes in the network. The closest meshes 

 and largest capillaries occur in secretory organs and in the lungs, which 

 are therefore abundantly supplied with blood. The muscles are well 

 supplied by slender capillaries in a rectangular meshwork. Serous mem- 

 branes and dense connective tissue have a scanty blood supply, from nar- 

 row capillaries in a coarse net. 



ARTERIES. The walls of the arteries are composed of three layers 

 the tunica intima, tunica media, and tunica externa, respectively. The 

 intima includes the endothelium and generally an underlying elastic 

 membrane, separated from the endothelium by a small amount of fibrous 

 tissue. The media is primarily a layer of circular smooth muscle fibers; 

 and the externa (formerly called the tunica adventitia) consists chiefly 

 of connective tissue. The thickness of all these layers is greatest toward 

 the heart. They become thinner at the places where the arteries branch, 

 and in the pre-capillary vessels nothing remains but the endothelium. 



The small terminal arteries are called arterioles. They are endothelial 

 tubes encircled by scattered smooth muscle fibers. In Fig. 163, C, the 

 oval nuclei of the endothelium are seen to be elongated parallel with the 

 course of the vessel. As is usually the case, the walls of the endothelial 

 cells are not visible. The rod-shaped nuclei of the muscle fibers are at 

 right angles with the axis of the vessel. In the somewhat larger artery, 



