290 NER VE-TISSUE. 



arise which reach the opposite horns, partly through the anterior 

 commissure, partly by running upward to the medulla oblongata, 

 where they decussate. The posterior roots also arise from two 

 bundles, one of which traverses the posterior commissure. Ac- 

 cording to Grerlach, it is also probable that from the reticulum of 

 the posterior horn nerve-fibers originate, which in this horn and 

 in the white substance take a centripetal course. The structure 

 of the posterior horn is considered to be mainly connective tissue, 

 especially in its gelatinous portion. 



(D) The white substance of the spinal cord, as well as that of 

 the cerebrum and cerebellum, is composed of medullated nerve- 

 fibers, which in the spinal cord run in a longitudinal direction 

 and furnish the outer investment with its nerve supply. This 

 substance borders on the anterior and posterior longitudinal 

 fissure, and is pierced by numerous offshoots of the pia mater, 

 which divide and subdivide the nerve-bundles into larger and 

 smaller groups. (See Fig. 121.) 



The anterior and posterior roots of the spinal nerves also pro- 

 duce smaller fissures in the lateral portions of the white sub- 

 stance, the sulcus lateralis anterior and posterior, by which the 

 white substance is divided on either side into an anterior, a lat- 

 eral, and a posterior cord. In the thoracic and cervical portions 

 of the spinal cord the posterior part is again divided into halves, 

 the middle division of which is termed the delicate cord, and the 

 larger lateral portions the wedge-shaped cords. 



The medullated nerve-fibers of the white substance vary 

 greatly in size. The coarser offshoots of the pia mater send lat- 

 eral prolongations between the nerve-fibers, every one of which 

 has a delicate investment of connective tissue the perineurium 

 internum or neuroglia. According to Gerlach, this connective 

 tissue is rich in elastic substance, and furnished with small glob- 

 ular or angular nuclei, but has a relatively scanty supply of 

 blood-vessels. 



In transverse sections of the white substance, we especially 

 recognize the medullated nerve-fibers by their graceful ensheath- 

 ing reticulum of connective tissue and their central, bright axis- 

 cylinder, which readily takes up the carmine stain. (See Fig. 

 122.) 



Each nerve-fiber is provided with a delicate outer investing 

 membrane the myeline sheath, or Schwann's sheath which 

 holds flat, oblong (in the transverse section spindle-shaped) 

 nuclei. Former observers denied the existence of this sheath ; 



