THE SYMPATHETIC SYSTEM. 509 



smaller arteries are more abundantly visible, their curvatures more 

 pronounced, and their pulsations more strongly marked. The super- 

 ficial veins are also increased in size and apparent numbers, and the 

 intervening tissues have a ruddy color, due to the abundant circulation 

 in their capillary vessels. This condition lasts for a certain time, while 

 secretion and digestion are in progress ; after which it gradually subsides, 

 and the circulation returns to its former state of comparative inactivity. 



It is evident that these reflex actions take place in some nervous cen- 

 tre, in which the centripetal impression is converted into a centrifugal 

 impulse. It appears that the ganglia of the sympathetic system act in 

 some measure as nervous centres for this purpose. This is indicated 

 by the fact that the vascular paralysis of the head and face, following 

 division of the sympathetic nerve in the neck, is more pronounced if 

 the superior cervical ganglion be extirpated ; and, as a general rule, 

 removal or destruction of the sympathetic ganglia produces more effect 

 than simple section of the nerve trunk. According to Yulpian, after 

 removal of the entire brain and the upper half of the spinal cord, 

 including the origin of the brachial nerves, in the frog, extirpation of 

 the cervical ganglion of the sympathetic is followed by vascular con- 

 gestion of the corresponding half of the tongue and buccal cavity. The 

 sympathetic ganglia have therefore a certain influence as the sources 

 of nervous power for vascular parts. 



But the action of these ganglia is limited in importance, and affects 

 only the parts to which their fibres are directly distributed. It has 

 already been shown that the roots of the sympathetic system emanate 

 from the spinal cord, and that they emerge from it at special points for 

 the head and limbs respectively. There is reason to believe that they 

 traverse the cord for some distance before detaching themselves from 

 its surface, and that their source in the gray substance is at a higher 

 level. According to numerous observers, a transverse section of the 

 cord in the cervical region causes marked vascular relaxation through- 

 out the body, as if all the vasomotor fibres had been divided in descend- 

 ing from above. This effect is produced by transverse sections of the 

 cerebro-spinal axis at any level in the cervical portion of the cord or in 

 the medulla oblongata, nearly to the posterior edge of the tubercula 

 quadrigemina ; but not by sections above that point. It is accordingly 

 maintained by some physiologists (Schiff, Owsjannikow, Liegeois) that 

 there is a common centre for all the vasomotor fibres of the body, 

 situated in the medulla oblongata or immediately above. In the opinion 

 of others (Brown-Se'quard, Yulpian) the vasomotor centres are more 

 widely scattered in the cerebro-spinal axis ; since reflex modifications 

 of vascularity may still be produced to some extent after division of 

 the spinal cord in the cervical region, and even certain lesions in the 

 cerebral hemispheres seem to produce vascular congestion in the limbs 

 or internal organs. This question is not positively determined ; but 

 it appears certain that the main centres of reflex action for the vascular 

 system are in the cerebro-spinal axis, whence their nerve fibres are 

 distributed, by various routes, to all parts of the body. 



