ARTERIES 171 



causes a spiral crumpling of certain muscle nuclei, the significance of 

 which has already been discussed (Fig. 106, p. 117). Between the muscle 

 fibers there are circular elastic fibers, or plates in the larger vessels, which 

 are thrown into wavy folds. Radial fibers, which connect these in a 

 general network, are slender and require special staining. White fibers 

 are present, apparently formed in considerable part by the muscle fibers 

 which they bind together. The proportion between the muscular and 

 elastic tissue in the media varies in different arteries. In the smaller 

 vessels, the muscular tissue predominates, and this is true also of the 

 cceliac, femoral and radial arteries. But in the common iliac, axillary 

 and carotid arteries the elastic tissue prevails, and in this respect they 

 resemble the largest arteries the aorta and pulmonary artery. 



The externa is a connective tissue layer which sometimes contains 

 scattered bundles of longitudinal muscle fibers. It has many longitudinal 

 elastic fibers, which are particularly numerous toward the media, where 

 they are often grouped as the external elastic membrane (Fig. 165). This 

 is not a fenestrated membrane, but is merely a dense zone of longitudinal 

 fibers. It is said to be well developed in the carotid, brachial, femoral, 

 cceliac and mesenteric arteries, and to be absent from the basilar and 

 other cerebral arteries. 



Nerves and vessels ramify in the externa. The walls of the larger 

 arteries are supplied with small blood vessels, the vasa vasorum, derived 

 from adjacent arteries. These are distributed chiefly to the externa; 

 they may penetrate the outer part of the media but do not reach the in- 

 tima. Lymphatic vessels form perivascular plexuses, and send branches 

 into the externa. The nerves are medullated and non-medullated. They 

 include vasomotor fibers which innervate the smooth muscle cells, and 

 sensory or afferent nerves which have terminal arborizations in the intima 

 and in the externa. Other nerve fibers end in lamellar corpuscles in the 

 externa of the aorta and other large vessels. 



Ganglia are not seen in the walls of the vessels, and the sympathetic 

 fibers to the muscles therefore travel considerable distances to their ter- 

 minations. In this respect the nerves to the smooth muscles of the ves- 

 sels differ from those to the musculature of the digestive tube. 



In the largest arteries (the aorta and pulmonary arteries) the intima 

 is very broad (Fig. 166), and it increases in thickness with age. Its en- 

 dothelial cells are less elongated than those of smaller arteries. They 

 rest on a fibrous subendothelial tissue, containing flattened stellate or 

 rounded cells, and networks of elastic tissue/- The elastic fibers are thicker 

 toward the media, finally producing a fenestrated membrane which corre- 

 sponds with the inner elastic membrane of smaller vessels, but which is 

 scarcely thicker than adjacent elastic lamellae. The broad media consists 

 of elastic membranes and muscle fibers, but the elastic tissue greatly 



