160 NERVOUS TISSUE. 



met with in conjunction with lecithin. Other substances which are 

 found in analyses of the nervous tissues are : — 



Cholesterin (C.-.^; Hj^^O) which was long considered to be a fatt)^ 

 body, but is prol)a1)ly more allied to the alcohols. It crystallizes in pearly 

 scales which are tinged blue by treatment with sulphuric acid and iodine. 



Extractive matters. — These prol)ably belong chiefly to the inter- 

 stitial fluid ; but, however this may be, they may be held to represent 

 the products of decomposition of the nervous substance. The following 

 have been recognised : — 



1. Lactic, formic, acetic, and (traces of) itric acid. 



2. Inosit. 



3. Krcntln. 



4. Ilijpo.vantlun. 



5. Lcncin (in tlie ox). 



Certain alljumhioid substances are also obtained from nervous tissue : they are 

 probably in great part derived from the nerve-cells and axial fibres. 



With regard to the reaction of nerve, the same law is said to jirevail as in 

 muscle — namely, that the substance of nerves in the living but quiescent state is 

 neutral, but becomes acid after death or prolonged excitement (Funke). The 

 saline or inorganic matters found by incineration are — phoHphortc acid. j>ho-\- 

 j)hatcs of alhdicn. which, as in muscle, largely predominate over other salts, ^^f^j'^/.vZ', 

 as a base, largely exceeding .sod(( ; carthij jjhoxjfhati:,':, in smaller proportion, 

 ■mafpu'sia prevailing over H)iu' ; i>lu»:phatc of iron ; chloride of sodium ; sulphate of 

 jyotash ; and a trace of silica. 



The white substance contains nearly 75 per cent, of M-ater ; the grey about 85 ; 

 the proportion of water is less in the spinal cord, and still less in the nerves. 

 Tlie fatty matters amount in the grey substance to nearly 5 and in the white to 

 nearly 1 5 per cent. ; in the nei-^'es the proportion fluctuates largely. It is worthy 

 of note that the brain, during embryonic and infantile life, contains much less fatty 

 matter and more water ; moreover, the grey and the white matter do not present 

 the same differences as in after life in the proi^ortions of water and fatty sub- 

 stance which they respectively contam. The brain of embryos of from ten to 

 twenty -two weeks has Ijeen foirnd to j-ield only from O'OD to 1'5 per cent, of fatty 

 substance ; that of the full-gro^^ir foetus from 3 to 4 per cent. 



VITAL PROPERTIES. 



The fibres of nerves are endowed with the property of transmitting 

 impressions, or, rather impulses, the effect of impressions, from the point 

 stimulated towards their central or their peripheral extremities. Certain 

 fibres are employed to conduct towards the nervous centres and are named 

 " afferent," others to conduct towards their distal extremities, which are 

 distributed in moving parts, and these fibres are named " eflTerent." 



The greater number of nerves possess both afferent and efferent 

 fibres, and are named compound or moto-sensory, innsmuch as they 

 minister both to sensation and motion. In such compound nerves the 

 two kinds of fibres are mixed together and bound up in tlie same 

 sheaths ; but in the most numerous and be-^t-known examples of this 

 class, the afferent and efferent fibres, though mixed in the trunk and 

 branches of the nerves, are separated at their roots. This is the case 

 in the spinal nerves : these have two roots, an anterior and posterior, 

 both for the most part consisting of many funiculi, and the posterior 

 passing through a ganglion with which the fibres of the anterior root 

 have no connection. Now it has been ascertained by appropriate ex])e- 

 riments on animals, that the anterior root is efferent and contains the 

 motor fibres, and that the posterior is afferent and contains the sensory 

 fibres. The fifth pair of cranial nerves has a sense ry root furnished 



