THE SPIXAL CORD AXD BRAIX 



811 



movement. Harrison's demonstration 1 is of great significance in connection with the "retraction 

 theory" and other ideas related to the neurone doctrine. 



2. In the Neural Crest Tissues. The nerve tissue elements of the sympathetic system and 

 of the ganglia of the cranial nerves and dorsal roots of the spinal nerves are derived from the 

 neural crest. Omitting, for the present, the development of the sympathetic system, it is found 

 that the cells of the paired masses which eventually become the cerebrospinal ganglia are at 

 first somewhat spherical, then oval in form, 

 sending out from either extremity or pole 

 a protoplasmic process. One process mi- 

 grates centrad, the other toward the tissues 

 of the periphery. The central process 

 penetrates the tissues of the neural tube 

 and, assuming the typical form of an axone 

 with its collaterals and end arborizations, 

 comes into contiguous association with cer- 

 tain cells of the central axis. The periph- 

 eral process is in reality an unusually 

 long dendrite, for it is centripetal in func- 

 tion; but owing to the fact that it is 

 usually provided with a myelin sheath it 

 is also termed the peripheral axone of an 

 afferent (or sensor) neurone. The central 

 processes of the cells of a single spinal nerve 

 ganglion form the dorsal nerve roots; the 

 peripheral processes constitute the afferent 

 portion of a spinal nerve. The cells them- 

 selves are transformed from bipolar into 

 apparently unipolar cells by the migration 

 of the cell body to one side and the con- 

 sequent approximation of the two pro- 

 teases to form a common pedicle in a 

 T-shaped manner (Fig. 586). 



Structure of the Nerve System. The 

 whole of the nerve system is composed of 

 nerve tissue and supporting connective 

 tissue. The neurones constitute the nerve 

 tissue, while the supporting tissue is com- 

 posed of the neuroglia and of white fibrous 

 tissue derived either from the investing 

 membrane or from the sheaths of its 

 numerous vascular channels. 



The Neurone. The neurone or nerve 

 cell element, whose individuality has 

 already been pointed out, exhibits remark- 

 able variations as to external characters, 

 dimensions, and form. The neurone pre- 

 sents a concentrated or swollen cell mass 

 and nucleus, formerly known as the nerve 

 cell (ganglion cell) and still retaining the 

 name! From this cell body are given off a 



number of processes of two distinct kinds: (1) protoplasmic processes which are commonly 

 branched and generally called the dendnCes; (2) a single, thinner, and paler process, the axone 

 ( ru-ix-cylindrr procex* ; Dritrrx' process; neuraxone; neurite). 



Varied Forms of Neurones. Bearing in mind that each neurone includes not only the cell 

 body and its dendritic processes, but also the axone or axis-cylinder process with all its ramifi- 

 cations, we may consider each of these divisions under separate heads. 



1. Nerve Cell Body. E.riprnnl Morphology. The bodies of nerve cells vary much in size, 

 measuring from 4 to 135 microns or more in diameter. The largest cells are found in the ventral 

 horns of the spinal cord, in the spinal ganglia, in the large pyramidal cell laver of the cere- 

 bral cortex, in the Purkinjean cell laver of the cerebellum, and in the column of Clarke 

 (dorsal nucleus) of the spinal cord. Very small cells are found in the olfactory bulbs, in the 

 granular layers of the cerebral and the cerebellar cortex, and in the gliosum cornualis of the cord. 



Although all nerve cells begin in the embryonic ectoderm as spherical germinal cells, they 

 later assume, in different regions, very different shapes. These external morphological relations 



FIG. 581. Showing some varieties of cell bodies of 

 neurones (diagrammatic.) A. Unipolar (amacrine) cell 

 from the retina. B. Bipolar cell from vestibular gang- 

 lion. C. Multipolar cell, with long axone, from spinal 

 cord. D. "Golgi cell," with short axone breaking up into 

 numerous terminal twigs. E. Pyramidal cell from cere- 

 bral cortex, a. Axone. clt. Collaterals, t. Telodendria. 



American Journal of Anatomy, June 1, 1907, vii, 1. (Anatomical Record, p. 116.) 



