826 



THE NERVE SYSTEM 



grams in the male and 40 grams in the female. The ratio of weight in proportion 

 to that of the brain among mammalia is lowest in the human species, being 1 

 to 51 in the male and 1 to 49.8 in the female. In the newborn the ratio is 1 tc 

 115. Its specific gravity is 1.038. 



External Morphology. In situ the spinal cord exhibits slight curvatures in the 

 sagittal plane, being convex ventrad in the transition from cervical to thoracic 

 portion and slightly concave ventrad 

 to the lumbar portion. Its position 

 varies also according to the degree of 

 curvature of the spinal column, being 

 raised slightly (a few millimeters only) 

 in flexion of the vertebral column. 



The intrinsically segmental nature 

 of the spinal cord is expressed by the 

 association of each definite segment 

 with the somatic segment supplied by 

 its nerves. Thirty-one pairs of spinal 

 nerves are commonly enumerated, 



XII. THORACIC- 



Thoracie 



V. LUMBAR 



COCCYGEAL- 



I Coccygeal 



FIG. 597. Showing the relations of the cord and 

 nerve origins to the levels at which the nerves emerge 

 through the intervertebral foramina (diagrammatic). 



FIG. 598. The cauda equina exposed within its 

 dural sheath. 



although two additional, rudimentary pairs, relics of a tailed vertebrate ancestry, 

 are demonstrable. 



The first pair of spinal nerves emerges between the occiput and atlas, and is 

 designated the first cervical; the other cervical pairs are named after the lower 

 of the two vertebrae which form the intervertebral foramen through which the 



