98 GENERAL ANATOMY OF THE TISSUES. 



substance contains a preponderance of albuminous matter, 

 besides a considerable quantity of fat. 



The physiological importance of the nervous tissue consists, 

 in the first place, in its subserving movement and sensa- 

 tion ; secondly, in its exerting a certain influence upon the 

 vegetative functions ; and thirdly, in its serving as a substratum 

 to the psychical activities ; in all which capacities, according to 

 what we know at present, the grey substance performs the more 

 important part, the white acting rather as a connecting conductor 

 between it and the organs. The nerve-cells are developed from 

 the common formative cells of the embrvo, whilst the nervous 

 tubules proceed from the coalescence of the membrane and 

 contents of many such cells, of a rounded, fusiform, or stellate 

 shape ; with this, in the medullary tubules a peculiar modifica- 

 tion of the contents occurs, in consequence of which it is divided 

 into a central solid filament and a softer investment. The 

 nutrition in the nervous tissue must be very active, especially 

 in the grey substance, as the great quantity of blood which 

 flows into it clearly shows, but the products of its decomposition 

 are wholly unknown. The white nervous substance is regene- 

 rated pretty readily in the peripheral nerves, and as it would 

 seem, in the spinal cord also. The adventitious formation of ner- 

 vous tubules has been observed in pathological, new formations, 

 and according to Yirchow's observations, it would even appear 

 that an abnormal development of grey substance may occur. 



The organs composed of nervous substance are : the peri- 

 pheral nerve-cords, nerve-membranes and nerve-tubules, the 

 ganglia, the spinal cord, and the brain. 



[Medullated nerve-fibres are found only in the Vertebrata, 

 and even in that class not in every division, as for example, in 

 Petromyzon (Stannius). Fibres without medulla always occur 

 together with the former, and in general in the same localities 

 as in man ; but in other situations also, as in the skin of the 

 Mammalia, in the electric organs of Fishes, and in the sympa- 

 thetic nerve of the Plagiostomata (Leydig). Where nerves are 

 found in the Invertebrata, they contain only pale fibres without 

 medulla, whose structure often completely resembles that of the 

 embryonic fibres of higher animals, especially as regards the 

 occurrence of great nucleated enlargements in the terminal ex- 



