482 



THE NERVOUS SYSTEM 



them are distributed to the white matter, but to some extent they 

 also supply the grey matter. 



The entire surface of the spinal cord presents, just beneath the 

 pia mater, a thin superficial layer or marginal veil of glia tissue. 

 In the brain this layer is somewhat exaggerated in thickness. 



THE REGIONS OF THE SPINAL CORD. The varying num- 

 ber of fibres which are given off at different parts of the spinal 

 cord results in considerable differences in size in its several por- 

 tions. By means of these peculiarities, as well as by the spinal 

 nerve roots to which they give origin, we distinguish a sacral, lum- 

 bar, thoracic, and cervical region. Each of these regions presents 

 certain more or less important morphological characteristics. 



In the sacral region the investment of white matter is very 

 thin, the grey matter though actually less in amount than in 



the more cephalad regions appearing 

 large by comparison. Both the ven- 

 tral and dorsal horns of grey matter 

 are short and thick. The substantia 

 gelatinosa of Rolando is of consider- 

 able volume. The cell groups in the 

 ventral horns of this region are a 

 ventro-medial and a dor so-lateral. 



The cord as a whole is small and 

 its transection nearly circular in out- 

 line. The five segments of this re- 

 gion contain the neurone centers for 

 the urinary bladder, the anus, some 



of the musculature of the lower limbs, and the sensory reflexes of 

 the perineum and genito-urinary organs. 



Below the sacral region the spinal cord tapers rapidly (conus 

 medullaris) and is continued downward for a considerable distance 

 as the filum terminate. The fibrous membranes which surround 

 the spinal cord continue even farther downward in the medullary 

 canal to form the central ligament, which is finally attached to 

 the sacrum or coccyx. , 



In the lumbar region there is a distinct enlargement,* chiefly 

 involving the grey substance, which here includes the immense 

 number of cells of the anterior horns whose " motor " fibres enter 

 the large lumbar nerve trunks for the supply of the lower limbs. 



* In those vertebrates which have no limbs e. g., the reptiles the cervical 

 and lumbar enlargements are not found. 



FIG. 377. TRANSECTION OF THE SPI- 

 NAL CORD OF A CHILD, THIRD 

 SACRAL SEGMENT. 



"Weigert stain, x 7. 



