416 



HISTOLOGY 



nerves. With the increase of myelin the sheaths become very slender and 

 can seldom be detected in the adult. It is ordinarily stated that the medul- 

 lated fibers of the central nervous system are without a neurolemma. 



Gray Substance. The gray substance consists of neuroglia, nerve cells, 

 and a confused mass of non-medullated nerve fibers running in all direc- 

 tions. The nerve 



Bloodvessels. *. , , cells ^ Q f 



types: (i) large 

 motor cells with pro- 

 cesses which enter the 

 peripheral nerves; (2) 

 cells with processes 

 limited to the central 

 nervous system and 

 extending through its 

 white substance from 

 one part to another; 

 and (3) small cells 

 with processes con- 

 fined to the gray sub- 

 stance. The neurax- 



ons of cells of the third type branch freely, and they may cross to the 



gray substance on the opposite side of the cord. 



The motor cells occur in groups in the ventral columns (horns). In 



the cervical and lumbar enlargements there are two groups, a ventro- 



medial and a dorso-lateral (Fig. 



424), which unite in the upper cer- 

 vical and thoracic portions of the 



cord; less well defined are the 



dorso-medial and ventro-lateral 



groups. In all of these groups 



the motor cells are large (67-135 ju 



Short rayed cells. Long rayed cells. 



FIG. 427. NEUROGLIA CELLS FROM THE BRAIN OF AN ADULT MAN 

 Golgi Method. X 280. 



my 



C ac 



FIG. 428. 



in diameter), with round or oval 

 nuclei and prominent nucleoli 

 (Figs. 429 and 430). Their proto- 

 plasm appears densely granular 

 in ordinary preparations, but when 

 specially treated it is seen to con- 

 tain an abundance of neurofibrils; 



if preserved in alcohol and stained with methylene blue, the groups of 

 granules known as Nissl's bodies may be demonstrated. As already 

 noted, these are abundant in vigorous cells but become reduced or dis- 

 appear in various conditions of exhaustion. Granules of brownish 



A, Neuroglia cells and nerve fibers from a crosa 

 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 cord 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. 



