112 GENERAL ANATOMY OF THE TISSUES. 



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 gray substance 

 performs the more important part, the white acting rather as a con- 

 necting conductor between j it and the organs. The nerve-cells are 

 developed from the common formative cells of the embryo, 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 modification 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 gray 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 

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

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

 tubules has been observed in pathological, new formations, and according 

 to Virchow's observations, it would even appear that an abnormal de- 

 velopment of gray substance may occur. 



The organs composed of nervous substance are : the peripheral 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 (Stan- 

 nius). 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 sympathetic nerve of the Plagiostomata (Leydig). Where 

 nerves are found in the Invertebrata, they contain only pale fibres with- 

 out 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 expansions, which re- 

 mains of the original formative cells, have, recently, less properly been 

 considered to be ganglion-globules. 



Literature. G. Valentin, " On the course and termination of the 

 nerves," in the "Nov. Act. Natur. Curios.," vol. xviii. t. i. ; R. Remak, 

 " Observations anatomies et microscop. de syst. nerv. struct.," Berol., 

 1838; A. Hannover, "Recherches microscopiques sur le systeme ner- 

 veux," Copenhague, 1844 ; R. Wagner, " Neue Unters. tiber den Bau und 

 die Endigungen der Nerven und die Structur der Ganglien," Leipzig, 

 1847; and "Neurologische Untersuchungen," in Gottingen "Anzeige," 

 1850; Bidder andReichert, "Zur Lehre voni Verhaltniss der Ganglien- 

 korper zuden Nervenfasern," Leipzig, 1847; Ch. Robin, in "1'Institut.," 



