THE NERVOUS SYSTEM. 801 



fibrils are sometimes continuous throughout a series of neurones. Nissl in a critical 

 review upon the -present status of the neurone theory announces his belief that the 

 entire neurone concept has been proved fallacious and must be abandoned. 



Admitting that the discovery of anastomoses between neurones and of the continu- 

 ity of the neuron* brils must be accepted as invalidating some of our ideas as to the sep- 

 arateness of the neurone units, it can still hardly be considered as overthrowing the 

 entire neurone theory. Embryologically, morphologically, and physiologically the 

 neurone shows a marked degree of individuality, and the fact that neurones sometimes, 

 usually, or even always anastomose, no more warrants the giving up of the neurone as 

 essentially a unit, than did the discovery of the fact that the protoplasm of epithelial 

 cells was continuous through intercellular "bridges" necessitate the giving up of the 

 epithelial cell as the structural unit of epithelium. 



The cell bodies of neurones are grouped mainly in the gray matter of the brain and 

 cord, in the ganglia of cranial, spinal, and sympathetic nerves, and in the peripheral 

 end organs of certain of the nerves of special sense. Protoplasmic processes ramify 

 mainly within the gray matter. 1 While some terminate in the gray matter in the imme- 

 diate vicinity of their cells of origin, it is the axones that make up the bulk of the white 

 matter of the brain, cord, and peripheral nerves. 



^'enroffliit.lu addition to the extensions inward of the pia mater and the connec- 

 tive tissue of the blood-vessels, there is found in both gray and white matter a tissue, 

 probably supportive in function, and peculiar to the nervous system the neuroglia. 

 Like the neurone, it originates in the epiblastic cells lining the embryonic neural canal. 

 These cells, at first morphologically identical, soon differentiate into neuroblasts, or 

 future neurones, and spongioblasts, or future neuroglia cells. In the adult two main 

 types of neuroglia cells are found, spider cells (Fig. 525), with spine-like, straight, 

 unbranching processes, and mossy (Fig. 526) cells with thick, rough, branching arms. 

 The former are found chiefly in the white matter, the latter in the gray matter in con- 

 nection with blood-vessels. As in the case of the nerve cell, the processes of these cells 

 do not anastomose, but interlace, forming a dense feltwork. By a special stain, Wci- 

 gert demonstrates neuroglia cell nuclei and separate fibrils. It seems probable that 

 this method fails to show the body of the cell, while staining its nuclei and the fibrils 

 which pass through it. 



It would thus appear that the architecture of the central nervous system, consid- 

 ered as an organ, is analogous to that of other organs. It has in the neurone its paren- 

 chyma, and this parenchyma is supported and bound together by a framework of con- 

 nective tissue. The essential difference lies in the fact that in the neurone is the highest 



rio. 525. NEU-ROGLIA CELL-SPIDER FIG. 52tt.-NErROGi.iA CELL -MOSSY TYPE- 



TYPE-HITMAN CEREBRUM. HIMAX CEREBRUM. 



morphological differentiation which protoplasm has attained, and representing chem- 

 ically the most complex molecules known. In the axone, sometimes a metre or more 

 in length, of a cell of microscopic dimensions, there is a distribution of cell protoplasm, 

 such as occurs in no other tissue or organ. The neuroglia also, while without question 

 a tissue supportive in function, is embryologically and morphologically different from 

 other forms of connective tissue. 



1 Exception, peripheral arm of spinal ganglion cell. 

 oJL 



