158 THE HUMAN BOD Y. 



cerelro-spindl liquid. The surface of the brain is folded 

 and the pia mater follows closely these folds; the arachnoid 

 often stretches across them : in the spaces thus left between 

 it and the pia mater is contained some of the cerebro-spinal 

 liquid. 



The Spinal Cord (Fig. 63) is nearly cylindrical in form, 

 being however a little wider from side to side than dorso- 

 ventrally, and tapering off at its posterior end. Its aver- 

 age diameter is about 19 millimeters (f inch) and its 

 length 0.43 meter (17 inches). It weighs 42.5 grams 

 (H ounces). There is no marked limit between the spinal 

 cord and the brain, the one passing gradually into the 

 other (Fig. 70*), but the cord is arbitrarily said to com- 

 mence opposite the outer margin of the foramen magnum : 

 from there it extends to the articulation between the first 

 and second lumbar vertebrae, where it narrows off to a. 

 slender filament, the filum terminate (cut off and repre- 

 sented separately at B' in Fig. 63), which runs back to the 

 end of the neural canal behind the sacrum. In its course 

 the cord presents two expansions, an upper, 10, the cer- 

 vical enlargement, reaching from the third cervical to the 

 first dorsal vertebrae, and a lower or lumbar enlargement^ 

 9, opposite the last dorsal vertebra. 



Running along the middle line on both the ventral and 

 the dorsal aspects of the cord is a groove, and a cross-sec- 

 tion show^s that these grooves are the surface indications of 

 fissures which extend deeply into the cord (C, Fig. 64) and 

 nearly divide it into right and left halves. 



The anterior fissure (1, Fig. 64) is wider and shallower 

 than the posterior, 2. The transverse section, (7, shows 

 also that the substance of the cord is not alike throughout, 

 but that its white superficial layers envelop a central gray 

 substance arranged somewhat in the form of a capital H. 

 Each half of the gray matter is crescent-shaped, and the 

 crescents are turned back to back and united across the 

 middle line by the gray commissure. The tips of each 

 crescent are called its horns or cornua, and the ventral, or 

 anterior cornu, on each side is thicker and larger than the 

 posterior. In the cervical and lumbar enlargements the 



*P. 169 



