THE SPINAL CORD 



535 



of the cord shows that it is composed of both white and gray matter, the 

 former covering the surface, the latter occupying the center. 



The Segmentation of the Spinal Cord. For the elucidation of many 

 problems connected with the physiologic actions of the spinal cord, as well as 

 of the symptoms which follow its pathologic impairment, it will be found 

 helpful to consider the cord as consisting physiologically of a series of segments 

 placed one above the other, the number of 

 segments corresponding to the number of 

 spinal nerves. Each spinal segment would 

 therefore comprise that portion of the cord to 

 which is attached a pair of spinal nerves. 

 The nerve-cells in each segment are in his- 

 tologic and physiologic relation with definite 

 areas of the body, embracing muscles, 

 glands, blood-vessels, skin, etc. 



If the exact distribution of the nerves 

 of any segment were known, its function 

 could be readily stated. By virtue of this 

 segmentation it becomes possible for each 

 segment to act independently of or in coopera- 

 tion with other segments near or remote, 

 with which they are associated by the intrinsic 

 or associative cells and their axons; and by 

 the same cooperative action the spinal cord 

 itself is enabled to act as a unit. 



THE STRUCTURE OF THE GRAY MATTER 



The gray matter extends throughout the 

 entire length of the cord in the form of two 

 crescentic-shaped columns, which are, how- 

 ever, united in the median line by a trans- 

 verse band or commissure forming a figure 

 resembling the letter H. Though varying 

 in shape in different regions of the cord, the 

 gray matter in all situations presents on 

 either side an anterior or ventral and a pos- 

 terior or dorsal horn. Between the two 

 horns there is a portion termed the inter- 

 mediate gray substance. The commissure 



presents in its center a narrow canal which extends throughout the entire 

 length of the cord. This canal is lined by cylindric epithelium and sur- 

 rounded by gelatinous material. (Fig. 223.) 



The ventral horn is short and broad and entirely surrounded by white 

 matter. The dorsal horn is narrow and elongated and extends quite 

 up to the surface of the cord, where it is capped by gelatinous matter, the 

 substantia gelatinosa. In the lower cervical and thoracic regions a portion 

 of the intermediate gray substance projects outward and forms the so-called 

 lateral horn. The gray matter fundamentally consists of a framework of 

 fine neuroglia supporting blood-vessels, lymphatics, medullated and non- 

 medullated nerves, and groups of nerve-cells. 



FIG. 223. SECTIONS THROUGH 

 DIFFERENT REGIONS OF THE 

 SPINAL CORD. A. At the level 

 of the sixth cervical nerve. B. At 

 the mid-dorsal region. C. At the 

 center of the lumbar enlargement, 

 i. Dorsal roots. 2. Ventral roots. 

 3. Dorsal fissure. 4. Ventral 

 fissure. 5. Central canal. 

 (Morris' "Anatomy ," after 

 Schwalbe.) 



