84 NORMAL HISTOLOGY. 



In its intimate structure the gray matter consists of multipolar 

 cells, neuroglia, and medullated fibres. The multipolar cells are of 

 two kinds motor cells and column cells. The motor cell has a 

 large body with long protoplasmic processes, the dendrites, and an 

 axis cylinder, which, emerging from the anterior cornu, becomes in- 

 vested with a medullary sheath, thus producing the axis cylinder 

 of a nerve fibre. The fibres of the anterior roots originate from the 

 motor cells. The column cells are smaller than those of the motor 

 type and have fewer dendrites. The axis cylinders from these cells 

 largely make up the white substance of the cord. The axis cylinders 

 of column cells send out many collaterals which penetrate the gray 

 matter. After entering the white matter, each cylinder divides into 

 two stem-fibres, ascending and descending, which extend longitudi- 

 nally through the cord, giving off many collaterals, which return 

 to the gray matter and terminate in tufts of fibrils, the stem-fibres 

 eventually terminating in the same way. The axis cylinders which 

 originate in Clark's column do not divide when they^ reach the white 

 substance, as do those of the column cells, but proceed upward to 

 the cerebellum. 



The supporting framework of the gray matter consists of neu- 

 roglia, which is composed of gliq cells and their processes. There 

 are two kinds of glia cells the ependymal cells and Deiter's cells. 

 Ependymal cells are of the epithelial type and line the lumen of the 

 central canal. They are provided with cilia and send out processes 

 into the surrounding tissues. Deiter's cells are found in the gray 

 matter, and afterwards in the white. They send out delicate proc- 

 esses, which form a supporting framework for the delicate nerve 

 structures. 



The investments of the spinal cord are the dura mater, on the 

 outside; the arachnoid membrane, centrally located; and the pia 

 mater, immediately surrounding the cord. From the pia mater, 

 septa of connective tissue extend inward. 



There are thirty-one pairs of nerves springing from the spinal 

 cord. The motor nerves are anterior, while the sensory nerves are 

 posterior. These nerves are distributed to the muscles, skin, a^d 

 other parts of the body, where they are provided with special ter- 

 minations adapted to receive impressions. 



