INNERVATION OF THE BLOOD-VESSELS OF DIFFERENT ORGANS 415 



vmc 



three neurons generally cover this entire distance. They form the 

 preganglionic path. Distally to these ganglia, the fibers constituting 

 the postganglionic path, either continue onward to different parts of 

 the sympathetic system, or reenter the spinal roots by way of the gray 

 rami communicantes where they intermingle with other efferent and 

 afferent fibers composing the different spinal nerves. In this way, even 

 the vasomotor mechanisms situated in the domain of the cerebrospinal 

 nerves, procure a sympathetic innervation 

 and are rendered, therefore, characteristic- 

 ally autonomic. 



The Location of the Motor End-organ 

 or Effector. — It has been proved histologic- 

 ally that the walls of the central arteries 

 contain much connective tissue and only a 

 relatively small number of smooth muscle 

 cells. In the peripheral arteries, on the 

 other hand, the latter are much more 

 numerous and are arranged here in the 

 form of a massive circular (tunica media) 

 and a thin longitudinal layer (tunica ex- 

 terna). No muscle tissue is present in the 

 true capillaries, these tubules being com- 

 posed solely of flat, nucleated epithelial 

 cells similar to those found in the intima of 

 the arteries. In the veins, connective tissue 

 predominates, while the muscular units are 

 poorly developed and not organized. Thus, 

 it happens that some of the veins possess 

 no muscle cells at all, while others, and 

 especially those of the lower extremities, 

 are equipped with only a very thin circular 

 layer of these cells. 



As the only effector present in the vas- 

 cular system is the smooth muscle cell, it 

 must be clear that vasomotor reactions 

 must be restricted to those channels which 



are actually in possession of these elements, namely the arteries and 

 certain veins. To be sure, it has been stated by Mall^ that the portal 

 vein receives a vasoconstrictor supply through the greater splanchnic 

 nerves, but these results have been shown by Burton-Opitz^ to be 

 based upon unsatisfactory experimental evidence. Thompson,^ how- 

 ever, has found that the stimulation of the sciatic nerve in dogs 

 and cats produces a visible constriction of the veins of the posterior 



Fig. 2 22. — Diagram to 

 Illustrate the Path Pursxjed 

 BY THE Vasomotor Fibers. 



SC, spinal cord; PR, its 

 posterior root ; AR, its anterior 

 root; Sn, spinal nerve; 5, sym- 

 pathetic ganglion; B, blood- 

 vessel; preganglionic path in 

 red; VMC, vasomotor center 

 P (red) white ramus; postgan- 

 glionic path in blue; P^, di- 

 rectly to blood-vessel; P, re- 

 current fiber, reentering spinal 

 nerve by way of gray ramus. 



lArchiv fiir Physiol., 1892, 409. 



2 Am. Jour, of Physiol., xxxvi, 1915, 325. 



Archly fur Physiol., 1893; also see: Bancroft, Am. Jour, of Physiol., i, 1898, 



477. 



