THE SYMPATHETIC SYSTEM. 



215 



cranial or spinal ganglia and in the brain or cord belong properly 

 to the cerebro-spinal system and not to the sympathetic. 



The efferent cerebro-spinal fibers which end in sympathetic 

 ganglia were given in the last chapter (p. 200) as the visceral 

 efferent fibers of the spinal nerves. Similar fibers occur hi the 

 cranial nerves in all vertebrates in which the sympathetic is devel- 

 oped in the head region. Where the head sympathetic is not devel- 

 oped the functions of the sympathetic, if performed at all, are prob- 

 ably performed by efferent visceral fibers in the cranial nerves with- 

 out the intervention of peripheral neurones. There is, of course, 

 the further alternative that there may exist peripheral ganglia 

 which are not developed from the cranial ganglia in the typical 

 manner. If in the lower fishes the head sympathetic is wanting 

 there can scarcely be any sharp distinction drawn between the 

 fibers which innervate smooth muscle and glands and those which 

 innervate the striated muscles of the gill arches. Wherever the 

 sympathetic is developed, however, such a distinction is very 

 clear, since in the one case a peripheral neurone is interpolated 

 in the reflex chain, in the other case not. The two sorts of fibers 

 take their origin from the same zone in the brain and spinal cord 

 but it happens that all the fibers which innervate striated muscle 

 are confined to the cranial nerves, including the spinal accessory, 

 while the sympathetic system extends through both head and trunk. 

 The nuclei of origin for fibers which innervate striated muscle 

 have probably become distinct from the nuclei of origin of sympa- 

 thetic efferent fibers (see Figure 107). An explanation of this 

 condition may be offered as follows. In primitive vertebrates 

 all visceral muscles were presumably non-striated and the visceral 

 reflex chain consisted of simple afferent and efferent limbs without 

 peripheral neurones. The more active branchial muscles became 

 striated and retained their direct innervation. For the inner- 

 vation of the non-striated muscle and glands neurones migrated 

 from the cranial and spinal ganglia and came to be interpolated 

 in the efferent pathway. As to the morphological status of these 

 migrated neurones of the sympathetic ganglia no sufficient expla- 

 nation can at present be given. As to the sensory neurones, it 

 would be not at all surprising that ganglion cells should migrate 



