THE STRUCTURE OF THE ELEMENTARY TISSUES. 109 



of the nervous system in accordance with the arrangement of the nerv- 

 ous structures about it. The cell is composed of granular protoplasm, 

 and lying in it is a large nucleus, -within which is a nucleolus. The 

 body of the cell is small in amount and proportion to the nucleus. 



Weigert has shown that the processes of the neuroglia-cells branch 

 and prolong themselves, forming in many places an extremely thick net- 

 work. These processes become changed in their chemical and physical 

 characters, so that they take a different stain from that of the cell-body 

 itself, and they thus form a really separate structure, distinct almost 

 from the mother-cell, just as the muscle tissue is distinct from its origi- 

 nal cell-protoplasm, or just as the substance of cartilage is distinct from 

 its original cell-body. While neuroglia-tissue is distributed throughout 

 the whole of the nervous centres, it is especially deposited in certain places. 

 It is found around the central canal of the spinal cord, and upon the 

 superficial surface of the spinal cord. It was formerly thought to com- 

 pose part of the gelatinous substance of Rolando in the spinal cord, but 

 this has been shown by Weigert not to be the case. 



In the brain a deposit of neuroglia is found beneath the ependymal 

 lining of the ventricles, and upon the superficial surface of the gray 

 matter of the cortex beneath thepia mater. It is distributed to some ex- 

 tent in all parts of the brain and spinal cord, but is not found in the 

 peripheral nerves. 



