THE SPINAL CORD. 503 



THE FUNCTIONS OF THE SPINAL CORD. 



Anatomic investigation has demonstrated that the spinal cord is com- 

 posed of a series of segments which are associated through their related 

 spinal nerves with the organs and tissues of definite areas of the body. 

 Physiologic investigation has also demonstrated that the segments by reason 

 of the presence of nerve-cells and nerve fibers may be regarded as composed 

 of: 



1. Nerve centers, each of which has certain special functions, and 



2. Conduction paths by which these centers are brought into relation not 



only with one another, but with the cerebrum and its subordinate or 

 underlying parts, e.g., the medulla oblongata, pons varolii and 

 cerebellum. 



A. THE SPINAL CORD SEGMENTS AS LOCAL NERVE CENTERS. 



The efferent cells of the spinal segments are the immediate sources of 

 the nerve energy that excites activity in skeletal muscles, glands, vascular, 

 and to some extent visceral muscles. 



The discharge of their energy may be caused : 



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



are surrounded or as the outcome of a reaction between the chemic 

 constituents of the lymph on the one hand and the chemic constituents 

 of the nerve-cell on the other hand. The excitation of the cell thus 

 occasioned is termed automatic or autochthonic excitation. 



2. By the arrival of nerve impulses, coming through afferent nerves from 



the general periphery, skin, mucous membrane, etc. 



3. By the arrival of nerve impulses descending the spinal cord from cells in 



the cortex of the cerebrum or subordinate regions. The excitation in 

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

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

 excitations the skeletal muscle movements are due to volitional, the 

 gland discharges and vascular and visceral muscle movements to emo- 

 tional phases of cerebral activity. 



Automatic Activity. By this expression is meant a discharge of energy 

 from the spinal nerve-cells occasioned by (a) a change in the chemic composi- 

 tion of the blood or lymph by which they are surrounded or probably a 

 reaction between the constituents of the lymph and the constituents of the 

 nerve-cell or (b) the development within the cell of a stimulus, the so- 

 called "inner stimulus," the outcome of metabolic activity. 



As no effect arises without a sufficient cause the term automatic has been 

 objected to and the term autochthonic has been suggested, as more nearly ex- 

 pressing the facts stated. A center so acting could not be regarded as prima- 

 rily a center for reflex activity, however much it might be influenced second- 

 arily by afferent nerve impulses. If the cell excitation is continuous though 

 variable from time to time, it is said to possess tonus and the organ or tissue 

 thus excited is also said to possess tonus or to be in a state of tonic activity. 

 If the cell discharge is intermittent in character it imparts to certain muscles, 

 e.g., the respiratory muscles, a rhythmic activity. It must, however, be kept 

 in mind that the tonus of nerve centers as well as of peripheral organs can also 

 be developed and maintained by the inflow of nerve impulses transmitted 



