260 



STUDIES IN PHYSIOLOGY 



Fia. 120. Portion of Spinal Cord, 

 magnified 30 times. 



Photographed through the microscope. 

 Upper three fourths of photograph is 

 gray matter containing nerve cells 

 (irregular black spots), with their 

 processes and axis cylinders (black 

 dots and lines) . Lower fourth of pic- 

 ture is white matter, showing five 

 bundles of nerve fibers. Between 

 these bundles are seen the cross sec- 

 tions of axis cylinders (small black 

 dots), each surrounded by medullary 

 sheath (white circle) . 



Nerve cells and proto- 

 plasmic processes are gray 

 in color, and since they 

 constitute the most impor- 

 tant part of the gray mat- 

 ter of the nervous system, 

 they give the latter its 

 characteristic shade. The 

 white color of the outer 

 portion of the spinal cord 

 is due to the presence 

 of the medullary sheaths, 

 which in a fresh condi- 

 tion are white and glis- 

 tening. 



Among the nerve cells 

 and fibers one finds a tis- 

 sue similar to connective 

 tissue and numerous blood 

 vessels ; the former serves 

 as a supporting frame- 

 work, the latter bring the 

 nutrients and oxygen that 

 are necessary for nervous 

 metabolism. 



2. ANATOMY OF THE SPINAL NERVES 



Number of Nerves. While studying the skeleton we 

 called attention to a row of holes on either side of the 

 spinal column. Through these openings between the verte- 

 brae (intervertebral foramina) there pass laterally from the 

 spinal cord the spinal nerves, of which there are thirty-one 

 pairs. These are arranged in five groups, named accord- 

 ing to the region of the vertebral column from which they 

 make their exit. The following table shows the relation 



