SECT. 31.] NERVOUS TISSUE. 71 



supplied with vessels than the white, and, in the peripheral ganglia, 

 contains different forms of connective tissue for enveloping its 

 different parts. 



The chemical composition of the nervous substances is still far 

 from being investigated with sufficient accuracy. In the white 

 substance, the central fibre of the nerve-tube consists of a 

 protein compound, very similar to the fibrin of muscle; the me- 

 dullary sheath, chiefly of different kinds of fats; and the envelope 

 of a substance similar to the sarcolemma. The grey substance 

 chiefly contains an albuminoid body, besides some fat. 



The physiological importance of the nervous tissne consists, 

 firstly, in its being the medium of motion and sensation; secondly, 

 in its exerting a certain influence upon the vegetative functions; 

 and thirdly, in serving as a substratum for the facilities of the 

 mind; in all which functions the grey substance appears, from all 

 that has been hitherto ascertained, to play the more important 

 part, the white serving more as a conducting medium between it 

 and the organs. 



The nerve-cells are developed from the ordinary formative cells 

 of the embryo and the nerve- tubes by the coalescence of numerous 

 similar cells, of a roundish, fusiform, or stellate shape, with en- 

 velope and contents ; to which, in the medullated tubes, a peculiar 

 alteration of their contents is superadded, in consequence of which 

 thev divide into a more consistent central filament, and a softer 

 sheath. The process of nutrition must, especially in the grey 

 substance, be very energetic, as the large quantity of blood which 

 flows to that substance sufficiently proves ; yet the products of 

 decomposition of this tissue are still wholly unknown. The white 

 nervous substance is readily enough regenerated in the peripheral 

 nerves, and, as appears, in the spinal cord also. Accidental for- 

 mations of nerve-tubes have been observed in pathological new 

 formations; nay, it even appears, according to an observation of 

 1 irchoic, that abnormal formations of grey substance may occur. 



The organs composed of nervous substance are : the peripheral 

 //< rriais cords, nervous membranes and nervous tubes, the gamjlia, 

 the spinal cord, and the brain. 



Literature. — G. Valextix, Ueber den Yerlauf unci die letzen Enden der 

 Nervt a, in Nov Act. Natur. Curios., vol. xviii. t. i. Remak, Observ. Anatomica; 

 et Mierosc. dc Stjst. Nerv. Struct., Berol., 1838 A. Haxxover, h'eeherehes 

 Microscopiques sur le Systbne Nerveux, Copenhagen, 1844. R. Wagxer, Neue 

 t nters. uber den Ban und die Endigungen der Nerven und die Structur der 

 Ganglien, Leipzig, 1847 ; and Neurol. Uhtersuchtrngen, in Gott. An-., 1850, 

 p. 54. Bidder and Reichert, Zur Lehre ram Verhdltniss der Gan^UenTwrper 



