008 PHYSIOLOGY. 



so related to one another as to subserve a certain function. These 

 cells give off numerous processes whereby they are brought into 

 direct communication with one another as well as other parts of the 

 body. These masses thus form structural integrations which per- 

 form corresponding integral functions. If at any time the struc- 

 ture suffers, the function must of necessity suffer also. 



The nerve-centers comprise the spinal cord, medulla oblongata, 

 pons Varolii, cerebrum, and cerebellum. 



COMMON PROPERTIES. There are certain properties which all 

 nerve-centers seem to possess in common and which are of interest 

 to the student : 



1. They all contain nerve-cells. These are the real centers of 

 activity. They both originate and conduct impulses. Nerve-fibers 

 are almost exclusively conductors. 



2. Nerve-centers are capable of discharging reflexes. They are 

 motor, secretory, and inhibitory reflexes. 



3. They are the seat of automatic excitement when phenomena 

 are manifested without the application of any apparent external 

 stimulus. 



4. The nerve-centers are trophic centers for both their nerves 

 and the tissues supplied by them. 



THE SPINAL CORD. 

 Structure of the Spinal Cord. 



"The key to the study of the central nervous system is to re- 

 member that it begins as an involution of the epiblast. It is origi- 

 nally tubular with a central canal whose brain-end is dilated into 

 ventricles. In the spinal cord there are three concentrated parts: 

 First, the columnar, ciliated epithelium; outside of this is the cen- 

 tral gray tube; and, covering all, the outer white, conducting fibers." 

 (Hill.) 



The spinal cord is that portion of the cerebrospinal axis which 

 is inclosed within the vertebral canal. It extends in the form of a 

 large, cylindrical cord from the upper level of the atlas to the first 

 or second lumbar vertebra. Above it is continuous with the medulla 

 oblongata. Below it becomes conical, to terminate finally in a slen- 

 der filament : the filum terminale. It is attached to the base of the 

 coccyx. The filum terminale emerges from the conical extremity of 

 the spinal cord. The cone is a mass of nerve-roots which, from its 

 striking resemblance to a horse's tail, has been termed the cauda equina. 







