VASCULAR NERVES FOR THE TRUNK AND LIMBS 



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but the fibrous tissue around the urethra is much weaker than around the 

 body of the penis, while around the glans there is none. The venous blood 

 is returned from the plexuses by comparatively small veins; all of which 

 are liable to the pressure of muscles when they leave the penis. The mus- 

 cles chiefly concerned in this action are the erector penis and accelerator 

 urinae. Erection results from the distention of the venous plexuses by a 

 sudden influx of blood resulting from the action of the nervous vascular re- 

 flexes. It is facilitated by the special muscular mechanism which prevents 

 the outflow of blood. 



The Vascular Nerves for the Trunk and Limbs. The skin and 

 muscles of the trunk receive their cutaneous and motor nerves by a seg- 

 mental arrangement in which the innervation is by bands corresponding 



FIG. 210. Plan of Distribution of Vaso-constrictor Nerves for the Fore Limbs. An. vi, 

 Annulus of Vieussens. (Modified from Moret.) 



with the segments of the cord and the spinal nerves. It is much the same 

 with the vascular nerves; they are distributed to the skin and walls of the 

 trunk in the same segment in which they arise. Langley says that the suc- 

 cessive bands overlap somewhat. 



In the fore legs or arms the vascular nerves arise from the first to the 

 fifth dorsal spinal nerves, run to the stellate ganglia, then by the gray rami 

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