OF THE ARTERIES. 281 



415. The nerves* of the arteries are derived from the spi- 

 nal marrow and the ganglia. The arteries of the organs of the 

 vegetative functions receive theirs from the ganglia, the others 

 from the spinal marrow. The nerves of the arteries form 

 around them a net-work analogous to those which the pneu- 

 mogastric nerves form around the resaphagus, and thus accom- 

 pany them into the interior of the organs. But, moreover, 

 some filaments terminate in the outer layer, and others reach 

 the middle membrane, on which they spread out into a very 

 delicate net-work. The former are soft and flat, the latter, 

 which are filiform and of extreme minuteness, have more con- 

 sistence, and pass through a shorter course. All the arteries 

 do not receive an' equal number of nerves; the pulmonary ar- 

 teries receive fewer than the aorta and its divisions. The 

 smaller the arteries are, the more abundant are the nerves. 

 The arteries of the brain are furnished with nerves only to 

 the place where they penetrate into the cerebral substance. 

 In old age, the nerves of the arteries, especially those of the 

 middle membrane, become less apparent. The great number 

 of nerves which are distributed to the arteries, shows that a 

 close connexion exists between the nervous system and the 

 circulatory apparatus, or between the nerves and the blood. 



416. The most physical properties of the arteries are the 

 toughness of their tissue, its tenacity and elasticity. It is to .the 

 firmness of the middle membrane that they especially owe the 

 power of preserving a considerable part of their tubular form, 

 even when empty of blood. Their specific gravity is about 

 108. Their thickness, which is generally considerable, is a 

 little augmented when they are empty. It is also somewhat 

 greater on the convex side of the curvatures than on the oppo- 

 site side, being nearly in the proportion of 8 to 7. It increases 

 proportionally to the caliber of the arteries in the same degree 

 as the latter diminishes; it is not the same, however, in all the 

 arteries of the same diameter; thus the parietes of the arte- 

 ries of the brain are very thin, and those of the limbs are 

 thick. 



* A. Wrisberg, loc. cil. Lucre, quadam nlserv. anaf. circa Ntrvos arfm'aa 

 adrnufcs ef comitantes. -Ito rum Jfg. Fruncof. ad Mnenum, 1810. 



