THE PHYSIOLOGY OF MOVEMENT 43 



The spinal cord is narrow and cylindric in shape and occupies the spinal 

 canal from the level of the first vertebra as far down as the second or third 

 lumbar vertebra. It presents both on its ventral and dorsal surfaces a 

 deep longitudinal fissure which partly divide the cord into halves, a right 

 and a left. 



Th3 Spinal Nerves. To each side of the spinal cord there are attached 

 thirty-one nerves, which as they pass out through foramina in the walls of 

 the spinal column are termed spinal nerves. Each spinal nerve is con- 

 nected with the spinal cord by two roots, termed from their relation to the 

 ventral and dorsal surfaces, the ventral and dorsal roots. A short distance 

 from the spinal cord these two roots unite to form the spinal nerve proper, 

 after which the fibers composing it pass outward to be distributed to various 

 peripheral organs. 



A transverse section of the spinal cord shows that each half is composed 

 externally of white matter, and internally of gray matter. The gray matter 

 in each half is arranged in the form somewhat of a crescent united in the 

 median line by a transverse band or commissure, the whole forming a 

 figure resembling the letter H. Though varying in shape in different 

 regions of the cord, the gray matter in all situations presents on either side 

 an anterior or ventral and a posterior or dorsal horn (Fig. 13). 



In the ventral horns of the gray matter are located large nerve-cells 

 which gives origin to nerve-fibers; these fibers early in embryologic develop- 

 ment become connected with structures for which they are by heredity 

 destined. With the growth of the embryo and the development of the 

 limbs there is a corresponding growth and development of the nerve-fibers 

 until they attain the size characteristic of the adult. The fibers collectively 

 constitute the ventral roots. Histologic investigation has shown that some 

 of the ventral roots contain two groups of nerve-fibers one of large and one 

 of small size. The fibers of large size pass forward and become directly 

 connected with the skeletal muscle. The fibers of small size -pass forward 

 but for a short distance, after which they leave the ventral root and enter a 

 group of nerve-cells (the so-called sympathetic ganglion) around the den- 

 drites of which their terminal branches are disposed; from these cells new 

 non-medullated fibers arise which pass backward into the spinal nerve and 

 in association with the fibers of large size pass to vascular muscleand gland 

 epithelium more especially of the skin and mucous membrane. In some 

 situations the fibers of small size pass through or alongside of these ganglia 

 to other ganglia more or less distant in which they terminate in a similar 

 manner; from the cells of these ganglia new non-medullated nerve-fibers 

 arise which pass directly to visceral and vascular muscles and perhaps to gland 

 epithelium. The two portions of these nerves of small size are known re- 

 spectively, as pre- and post-ganglionic. 



The nerve-fibers of small size found in the ventral roots of the spinal 

 nerves and in some of the encephalic nerves and distributed by way of 

 the ganglia to visceral and vascular muscles and to gland epithelium con- 

 stitute a system of nerves termed the autonomic system (see Chapter XXVI) . 



The dorsal root fibers originate from cells situated outside of, but 

 derived from the spinal cord. The cells in this situation at first develop two 

 processes from opposite ends, which at a later period shift their position, 

 unite and form a single process after which, a division into two branches 



