452 TEXT-BOOK OF PHYSIOLOGY. 



FUNCTIONS OF THE SPINAL CORD. 



Physiologic investigation has demonstrated that the spinal cord, 

 in virtue of the presence of nerve-cells and nerve-fibers, may be re- 

 garded as composed of: 



1. Independent nerve-centers, each of which has a special function; 



and 



2. Conducting patjjs by which these centers are brought into relation 



with one another and with the cerebrum and its subordinate 

 or underlying parts. 



The cord, moreover, may be considered as consisting physiologically 

 of a series of segments placed one above the other, the number of 

 segments corresponding to the number of spinal nerves. In other 

 words, a spinal segment comprises that portion of the .cord to which 

 is attached a pair of spinal nerves. The nerve-cells in each segment 

 are in histologic and physiologic relation with definite areas of the 

 body, embracing muscles, blood-vessels, glands, skin, etc. 



The Spinal Cord as an Independent Center. The efferent 

 cells of the spinal segments are the immediate sources of the nerve 

 energy which excites activity in muscles, blood-vessels, glands. The 

 discharge of their energy may be caused: 



1. By variations in the composition of the blood or lymph by which 



they are surrounded. The activity of the cell thus occasioned 

 is termed automatic or autochthonic (Gad). 



2. By the arrival of nerve energy coming through afferent nerves 



from the general sentient periphery, skin, mucous membrane, etc. 



3. By the arrival of nerve energy descending the spinal cord from 



the cerebrum or subordinate structures. The peripheral activity 

 in the former instance is said to be reflex or peripheral in origin; 

 in the latter instance, direct or cerebral in origin. In this latter 

 instance, also, the muscle movements are due to volitional, the 

 vascular variations and glandular discharges to emotional, 

 forms of cerebral activity. 



Each segment of the spinal cord may be regarded, therefore, 

 because of its contained nerve-cells: 



1. As a center for automatic activity. 



2. As a center for the reception of excitations arising either at the 



periphery or in the cerebrum, and for their subsequent trans- 

 mission through efferent nerves to various peripheral organs. 

 Automatism. The growth, the nutrition and multiplication of 

 the cells of various tissues, and their continuous and rhythmic activity, 

 have been attributed to an automatic action of the spinal nerve- 

 cells. By ^ this expression is meant a discharge of energy from the 

 cells occasioned by a change in their environment, i. e., in the chemic 

 composition of the blood or lymph by which they are surrounded, 

 and independent of any excitation coming through afferent nerves. 



