CHAPTEE IV. 



THE SPINAL COED. 



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

 contained within 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 con- 

 tinuations of which make connection with those of the brain above ; and 

 internally of gray substance arranged in two symmetrical bands occupy- 

 ing the central portions of its right and left lateral halves. It is so 

 constituted, therefore, as to act in a double capacity: First, as an organ 

 of nervous communication between the brain and the external parts ; 

 and secondly, as an independent nervous centre, with endowments and 

 functions of its own. 



Arrangement of Gray and White Substance in the Spinal Cord, 



The mutual relations of the gray and white substance form the neces- 

 sary basis for a complete physiological anatomy of this part of the 

 nervous system. The connections of the nerve fibres with the cells of 

 the gray substance and with various parts of the longitudinal columns, 

 as well as those of the different nerve cells with each other, are the most 

 important for this purpose. It has not yet been possible to make out 

 these connections with certainty for all parts of the cord ; but much has 

 been accomplished in this respect by the examination of microscopic 

 sections made in various directions, after hardening the tissues of the 

 cord in alcohol or in weak solutions of chromic acid or potassium bichro- 

 mate, and by making the fibres and cells more distinct by means of 

 staining preparations. With regard to the relative proportions, in dif- 

 ferent parts of the cord, of its two constituent elements, it is evident, 

 as shown by Kolliker and Gerlach, that the gray substance is increased 

 in quantity at the situation of the cervical and lumbar enlargements, 

 and that the white substance, on the other hand, diminishes gradually, 

 from its upper to its lower extremity. This fact corresponds with the 

 known physiological relations of the cord ; namely, that by its gray sub- 

 stance it acts as a nervous centre for the corresponding regions of the 

 body ; and also that the fibres of its white substance form communica- 

 tions between the parts above and the spinal nerves which are given 

 off below. 



The Or ay Substance. The gray substance in the spinal cord, as 

 elsewhere, consists of a mixture of nerve cells and nerve fibres, of which 

 the nerve cells are the peculiar and distinctive element. They are all 



(443) 



