824 



THE NERVE SYSTEM 



sively conducting substance, the gray is not exclusively ganglionic, for the former 

 encroaches on the latter; in some localities, as in the ventral horns of the spinal 

 gray, in parts of the cerebral cortex, in the reticular formation of the pons and 

 medulla oblongata, and in the column of Clarke (dorsal nucleus), the admixture 

 of myelinic fibres is considerable. Both white and gray substance is pervaded 

 by the neuroglia. 



The specific gravity of the cortical gray substance is 1.021; of the great ganglia, 

 1.034; of the gray substance in the cerebellum and mesencephalon, 1.040; and of 

 the white substance, 1.028. 



For convenience of study, and somewhat in correspondence with phyletic 

 development, the central axis of the nerve system is divided into (1) the spinal 

 cord and (2) the brain, grossly subdivided into (a) medulla oblongata, pons, and 

 cerebellum; (b) mid-brain; and (c) fore-brain. This gross subdivision is arbitrary 

 and the interrelations of the parts would be obscured were too much stress laid 

 upon any mode of separation. 



THE SPINAL CORD (MYELON; MEDULLA SPINALIS). 



The spinal cord is the attenuated, nearly cylindrical part of the cerebrospinal 

 axis which lies in the vertebral canal, occupying its upper two-thirds in the adult. 



It extends from about the level of the 

 atlooccipital articulation (or lower bor- 

 der of the pyramid decussation) to the 

 level of the lower border of the body 

 of the first lumbar vertebra, where it 

 terminates in a slender filament of 

 gray substance enveloped by pia, and, 

 further caudad, by a sheath of dura 

 which is attached to the dorsum of the 

 coccyx. The spinal cord is continuous 



ARTERIA 

 VERTEBRALIS 



N.CERV. I. 



;\ I V LIGAMENTUM 



DENTICULATUM 



Fici. 595. Showing the relation of the spinal cord 

 to the dorsal surface of the trunk. The vertebra- are 

 shown in red outlines. 



FIG. 596. Ventral view of medulla oblonga!..' 

 and upper part of spinal cord. Dura and arach- 

 noid cut along median line and folded aside. A 

 and R are fairly constant velar folds of the arach- 

 noid. (After Key and Retzius.) 



