CHAPTER IV. 

 TJJJ SPINAL CORD. 



THE spinal cord is that part of the cerebro-spinal system contained 

 in the spinal canal, and which sends its nerves to the muscles and 

 integument of the trunk and limbs. It consists externally of white 

 substance, forming longitudinal tracts of nerve fibres, the continuations 

 of which make connection with the brain above ; and internally of gray 

 substance surrounding its central canal, and occupying the interior of 

 its lateral halves. It is therefore constituted to act in a double capac- 

 ity : First, as a medium of communication between the brain and the 

 peripheral organs ; and secondly, as an independent nervous centre, 

 with special endowments of its own. 



Arrangement of Gray and White Substance in the Spinal Cord. 



The relations of the gray and white substance form the necessary 

 basis for a physiological anatomy of this part of the nervous system. 

 The connections of the nerve fibres with the gray substance, and their 

 course in the longitudinal columns, are the most important for this 

 purpose. These relations are not fully known ; but much has been 

 accomplished in this respect by the examination of transverse and lon- 

 gitudinal sections of the cord, either in the fresh condition or after 

 the use of hardening and staining preparations. The size and form 

 of the cord, as well the quantity and configuration of its white and 

 gray substance, vary much in its different parts. In the upper cervical 

 region, it is nearly cylindrical ; at the cervical enlargement, it is 

 widened laterally, and flattened in an antero-posterior direction ; in 

 the dorsal region, it again approximates the cylindrical form, but is 

 reduced in size ; its second enlargement is at the beginning of the 

 lumbar region ; after which it diminishes rapidly to its termination. 

 It is evident from an inspection of its section surfaces at different 

 levels, that the cervical and lumbar enlargements are mainly due to 

 an increased quantity of gray substance in these regions j and that the 

 w T hite substance, on the whole, diminishes gradually from above down- 

 ward. This agrees with the double physiological character of the cord; 

 its gray substance acting as a nervous centre for the -corresponding 

 regions of the body, while its white substance, at any one point, rep- 

 resents the tracts of communication for nerves given off below. 



The Gray Substance. The gray substance in the spinal cord, as 

 elsewhere, is a mixture of nerve cells and nerve fibres, of which the 

 nerve cells are the distinctive element. They are all provided with 



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