PACINIAN CORPUSCLES. 345 



The nerve-fibre is single as it runs along the peduncle, unless when the latter 

 supports two corpuscles ; it retains the medullary sheath until it reaches the core, 

 into which the axis-cylinder alone passes, freed from its primitive and medullary 

 sheaths. In its course through the core it is somewhat flattened, and presents the 

 appearance either of a pale, finely striated, and very faintly outlined band or stripe, 

 or of a darker and more sharply defined narrow line ; differing thus in appearance 

 according as its flat side or its edge is turned towards the eye. The contrast in the 

 appearance of the fibre before and after entering the core is well exhibited after 

 treatment with osmic acid, which stains the medullary sheath deeply, whereas the 

 axis-cylinder is far less stained. It sometimes happens that the fibre regains its 

 double contour for a short space, and changes again before it terminates ; this is 

 especially liable to occur while it passes through a sharp flexure in a crooked core. 

 The fibre usually ends by a sort of knob at the further extremity of the core, which 

 is here itself somewhat dilated. The knob, often finely granular, appears to be an 

 expansion of the axis-cylinder, and is sometimes of considerable size. It may be of 

 an irregular shape with processes branching outwards from the sides, and in such 

 cases has been taken to represent a nerve-cell ; but the characteristic nucleus of the 

 latter is absent. The ultimate destination of the processes is unknown. The axis- 

 cylinder shows the usual longitudinal fibrillation as it passes through the core, and 

 the fibrils become somewhat spread out as they pass into the terminal expansion. 

 In many cases the fibre, either immediately before terminating, or in its course 

 through the core, divides into branches. In case of division of the fibre, the core 

 is generally, but not invariably, divided in a corresponding measure, and the 

 inner tunics present a figure in keeping with it. It is worthy of remark, that the 

 nerve-fibre in its course along the core runs almost exactly in the axis of the latter, 

 and it maintains this position even when passing through the abrupt flexures of an 

 irregularly shaped core. It sometimes happens that a fibre passes quite through one 

 corpuscle and terminates in a second, resuming its original size and dark outline 

 while passing from the one to the other. A little artery enters the Pacinian 

 body along with the nerve, and soon divides into capillary branches, which run up 

 between the tunics. They then form loops, and return by a similar route into a vein 

 corresponding to the artery : a single capillary usually accompanies the nerve as far 

 as the core, and passes some way on the wall of the latter, sometimes with a spiral 

 direction (Bowman). Occasionally a vessel enters the corpuscle at the distal end 

 and passes towards the core, uniting the tunics in its passage. 



As to the nature of the core of the Pacinian body, there is considerable difference of 

 opinion. That it is not merely an expansion of the medullary sheath of the nerve-fibre, as 

 was thought by Engelmann, is shown by its behaviour with staining fluids, and particularly 

 osmic acid (see fig. 406). Moreover in cases where the medullary sheath is prolonged for some 

 distance to the core, as occasionally happens, the contrast between it and the substance which 

 surrounds it, is very marked. 



In considering the true nature of the core, it should first be remarked that it is not 

 completely homogeneous and structureless, as on superficial examination it seems to be, but 

 exhibits at least in its outer part longitudinal striation and nuclei in variable number. In 

 transverse section the striation in the outer part of the core is seen to be concentric, and pro- 

 duced apparently by flattened nucleated cells, which are so arranged as to inclose the inner 

 and more homogeneous portion. At the entrance of the nerve-fibre into the core the nucleated 

 cells here spoken of are to all appearance continuous with a layer of cells in the endoneurium 

 around the entering nerve-fibre, so that this outer part of the core, at least, might be regarded 

 as formed by an expansion of endoneurium. The inner part, on the other hand, that, namely, 

 which is in immediate contact with the axis-cylinder, appears structureless. In its behaviour 

 towards staining re-agents, it resembles protoplasm, and it is possible that it may represent the 

 protoplasmic layer which in young nerves intervenes between the axis-cylinder and the sheath 

 of Schwann of a nerve-fibre, and in which the fatty substance of the medullary sheath 

 becomes deposited. 



Nothing positive is known concerning the special purpose in the animal economy which 



