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TEXT-BOOK OF PHYSIOLOGY 



2. The comma tract, a narrow bundle of fibers situated in the anterior 

 portion of the column of Burdach. When it is divided it degenerates down- 

 ward. 



3. The septo-marginal tract, an oval-shaped tract situated along the 

 margin of the posterior longitudinal fissure. 



4. The cornu-commissural tract found along the border of the anterior 

 portion of the posterior column as far forward as the posterior commissure. 

 Both of these tracts are best developed in the lumbosacral region. They 

 arise from nerve-cells in the gray matter. They undergo descending 

 degeneration when divided, but not after division pf the dorsal roots. 



THE FUNCTIONS OF THE SPINAL CORD 



Anatomic investigation has demonstrated that the segments of the spinal 

 cord 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 instance 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 psychic 

 states of a volitional, or an affective or emotional character; the gland dis- 

 charges and vascular and visceral muscle movements to affective or emotional 

 phases of cerebral activity only. 



Automatic Excitation. By this expression is meant a discharge of 

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

 composition of the blood or lymph by which they are surrounded or prob- 

 ably 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 



