CENTEAL OEGAN AND PEEIPHEEAL NEEVES (PHYSIOLOGICAL). 37 



the fibers of the peripheral nerves have two quite-different origins. All 

 motor fibers arise as axis-cylinder processes, or neuraxons, from the cells 

 lying in the ventral portion of this canal. Each cell sends out a fibril 

 toward the surface, and there the fibrils approximate each other to form 

 ventral nerve-roots. The sensory root-fibers, which arise mainly dorsal, have 

 an entirely different beginning. They proceed from the ganglia which 

 lie near the cord throughout its length, and not from within the cord itself. 

 Prom the cells of these ganglia (spinal ganglia and ganglia of the cranial 

 nerves) fibers grow in two directions. One set enters the central organ, 

 the other grows toward the periphery as sensory nerves. 



In the vertebrates the cells of origin of most motor nerves, especially 

 those supplying striated muscle, are in the central axis. They have already 

 been considered as forming good examples of the superposition of different 

 neurons. But not all of the motor nerves arise in this way. Scattered 

 throughout the body we find ganglion-cells, whose axones end in the non- 

 striated muscle-fibers of the blood-vessels, the intestines, the heart, and 

 other viscera. These cells, usually classed as belonging to the sympathetic, 

 must be regarded as motor cells, because on their normal supply depends 

 the inherent power of contraction which these organs possess. They lie in 

 many locations — for instance, in the intestinal walls and the heart — in 

 relatively close contact with other axones which arise from other places, as 

 from the spinal cord, etc. Here, too, then, in the sympathetic there are 

 motor paths of different orders. We have seen that in mammals a large 

 share of the secondary motor paths reach, in some way, to the organs of 

 consciousness. That is not true of all these tracts. It is better to consider 

 the motor centers, the central and peripheral sympathetic as capable of in- 

 dependent action, and to determine in each case how far higher nerve-tracts 

 associate themselves to these, and how far higher nerve-centers can affect 

 their action. With mammals all the striated muscles are innervated from 

 the central organs, and only the smooth muscles, as well as those of the 

 heart, are to some extent independent of them; but with the lower animals 

 there are also, in the periphery, many ganglion-cells for voluntary muscles. 



The sensory nerves in vertebrates are mainly outgrowths from the 

 cells of the spinal ganglia. They also split up when they arrive at the pe- 

 riphery, and end either freely in the epithelium or in some modified 

 end-apparatus, usually an epithelial structure. Aside from the ontogeny 

 of the sensory nerves, much of interest is known of their phylogeny. 

 As is well-known, the outer covering of slightly-developed animals, as the 

 eoelenterates, among the ordinary epithelial cells, presents still others 

 characterized by their arrangement in groups and by the possession of 

 a long end-filament, which sinks into the nervous system. In the whole 

 list of lower animals it is a frequent occurrence that cells lying in the 



