WHITE AND GRAY SUBSTANCE. 



IIQ 



figure shows a prolongation of the pia mater, containing blood vessels, 

 into the white substance. It has not been established beyond doubt 

 that such ingrowths of connective 



. . , i , f White External limiting 



tissue may not take part in form- substance, membrane. 



ing supporting tissue around the 



nerves. 



The nerve fibers of the white 

 substance vary in diameter, the 

 coarsest being found in the ven- 

 tral and the lateral parts of the 

 dorsal funiculi; the finest are in 

 the median parts of the dorsal 

 and lateral funiculi. Elsewhere 

 coarse and fine ones are inter- 

 mingled. Their general direc- 

 tion is parallel with the long axis 

 of the cord. Like other nerve 

 fibers they consist of neuroplasm 

 and nbrillae. Most of them are 

 medullated and in cross section 

 the myelin often forms concen- 

 tric rings. Although a few 

 observers have described nodes it is generally considered that there are no 

 nodes in the central nervous system. During the development of the 



myelin, fibers have been found en- 

 circled by sheath cells, Fig. 144, B. 

 In longitudinal view, these sheath 

 cells are seen in depressions of the 

 myelin, where they greatly resem- 

 ble the neurolemma cells of per- 

 ipheral nerves. With the increase 

 of myelin they become very slender 

 and can seldom be detected in th*e 

 adult. It is ordinarily stated that 

 the medullated fibers of the central 

 nervous system are without a neuro- 

 lemma. 



The gray substance [matter] is 

 composed of a neuroglia framework 



containing capillary blood vessels and some larger ones, together with the 

 cell bodies and non-medullated processes of many nerve cells. The 



Cross sections of 

 medullated 

 nerve fibers 

 consisting of 



Axis cylinder 



and 

 ^Medullary sheath. 



. Xeuroglia cells. 



Connective tissue. 



Blood vessels. 



FIG. 143. FROM A CROSS SECTION OF THE HUMAN 

 SPINAL CORD IN THE REGION OF THE LATERAL 

 FUNICULUS. X 180. 



my 



FIG. 144. 



A, Neuroglia cells and nerve fibers from a cross 

 section of the spinal cord of an elephant. B, 

 Neuroglia cells, nerve fibers and sheath cells, 

 from the spinal cord of a pig, 2 weeks after 

 birth. C, Isolated fiber from the eord of 21 

 cm. pig embryo, stained with osmic acid. 

 (After Hardesty.) a. c., Axis cylinder; my., 

 myelin; n., neuroglia nuclei; n. f., neuroglia 

 fibrils; s. c., sheath cell. 



