214 NERVOUS SYSTEM OF VERTEBRATES. 



cells have neurites which break up into branches within the spinal 

 ganglion and form pericellular baskets about the bodies of 

 ordinary spinal ganglion cells. The functions and the structural 

 arrangement of the sensory sympathetic cells and their supposed 

 connection with the spinal ganglion cells require further study. 



The statement is made (Onuf) that fibers which enter the 

 visceral sensory column of the spinal cord are caused to degenerate 

 by cutting the rami communicantes of the sympathetic. This 

 would indicate that sensory cells situated in the sympathetic 

 system send their neurites directly into the spinal cord. Such 

 cells and fibers are not shown in Figure io8. 



Another form of peripheral reflex has been suggested in which 

 the branches of a single neurone only would be involved. It is 

 supposed that an impulse may travel from a peripheral ganglion 

 back along an efferent fiber and go out from it along a collateral 

 to stimulate a sympathetic excitatory cell. This form of reflex 

 is iUustrated in Figure no. It must be said that this hypothesis 

 seems very improbable in view of what we know of the polarity 

 of neurones in other parts of the nervous system, and that there is 

 little direct evidence in its support. 



The essential feature of the sympathetic system is that in the 

 visceral reflexes governing smooth muscle, heart muscle and glands,, 

 there is interpolated in the efferent limb of the reflex chain a per- 

 ipheral neurone between the cerebro-spinal fiber and the organ 

 innervated. There may be two such neurones interpolated and 

 the sympathetic may carry out peripheral reflexes without the 

 aid of cerebro-spinal elements, but these things are still uncertain, 

 as are also the sensory sympathetic neurones. The sympathetic 

 system does not to any great extent carry on independent or 

 automatic functions. The great majority of its actions are directly 

 aroused by efferent impulses coming from the brain or spinal 

 cord and in response to the stimulation of visceral sensory fibers 

 which rim through, but have no connection with, the sympathetic 

 ganglia. In a strict sense the sympathetic consists solely of the 

 neurones whose cell-bodies lie in the various gangha, the excitatory 

 and sensory sympathetic cells above described. The afferent 

 and efferent neurones whose cell-bodies lie respectively in the 



