248 



THE CIRCULATION OF THE BLOOD 



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 

 back through the ramus vertebralis to join those cervical nerves that enter 

 into the brachial plexus, figure 211. 



FIG. 211. Plan of Distribution of Vaso-constrictor Nerves for the Fore Limbs. 

 (Modified from Moret.) 



The nerves for the blood vessels of the lower limbs arise from the tenth 

 dorsal to the second lumbar nerves. These pass to the ganglionic chain, and 

 gray rami are given off which join the lumbo- sacral plexus and run with the 

 divisions of that nerve complex to their distribution in the skin and muscles. 

 Vaso-constrictors and vaso-dilators have a common course to the lower limbs. 



The Vaso-constrictor Nerves for the Veins. Mall has proven that 

 vaso-constrictors are present for the portal vein. These fibers are present 

 in the splanchnic nerves. Other evidences have been observed which 

 render the view probable that vaso-motors for the veins in general exist. 

 Hough, for example, in an extended study of the capillary pressure found 

 many variations which were readily explained only on the assumption of 

 veno- motor activity, see figure 201. 



