182 ANIMAL PHYSIOLOGY. 



coagulated, white substance of Schwann, and from its marrow- 

 like consistency, the medullary sheath. The limiting mem- 

 brane is absent from the fibres within the brain and spinal 

 cord. The axis-cylinder is of albuminoid composition, while 

 the white substance is rich in protagon, a material containing 

 phosphorus, soluble in warm alcohol, but not in ether, and 

 readily yielding fatty compounds by decomposition. Within 

 its nervous centre a medullated fibre often becomes much 

 reduced in diameter, and at its peripheral termination it may 

 consist of axis-cylinder without any white substance, and 

 may break up into branches. The axis-cylinder, when care- 

 fully examined, exhibits a longitudinal striation, considered 

 by some observers as indicating a bundle of primitive fibrils 

 which, by separating, give the appearance of branching 

 referred to. It is, however, to be remarked that by the 

 nitrate of silver method, a transverse striation may be like- 

 wise exhibited; and as in the case of muscular fibre, so also 

 in that of axis-cylinders, it is doubtful what importance is to 

 be attached to either longitudinal or transverse markings. 



Another set of iion-medullated fibres of soft consistence, 

 darker than the medullated fibres, distinguished as grey fibres, 

 and presenting large nuclei, which occupy their whole 

 breadth, are found mingled with medullated fibres most 

 abundantly in the sympathetic system, and also constitute 

 all the filaments of the nerve of smell. A doubt may be 

 entertained as to whether the nuclei are imbedded in their 

 protoplasmic substance, or belong to a sheath. But what is 

 most important for the student to understand is, that every 

 nerve in the body has an albuminoid thread, while only some 

 are enveloped in the substance yielding protagon. 



137. Nerve-corpuscles, improperly called nerve -cells, are 

 sometimes surrounded with a nucleated sheath, but they 

 have no cell wall. They consist of a clear nucleus, and one 

 or more nucleoli imbedded in a mass of protoplasm loaded 

 with granules. They send out, in probably every instance, 

 processes termed poles. Some of these poles are continuous 

 with nerves, others communicate with other corpuscles; while, 

 in the case of multipolar corpuscles, many of them would 

 appear to ramify till the branches reach an extreme tenuity. 



The firmness of the different parts of the nervous system 



