210 



INNEllVATION. 



[CHAP. vni. 



Fig. 53. 



form of oil-globules. The interior of the tube is also rendered 

 decidely granular. In water the white substance of Schwann 



remains undissolved, while the interior of 

 the fibre is frequently, though not always, 

 rendered granular. 



The tubular membrane presents the 

 same general characters wherever it is met 

 with. But the white substance of Schwann 

 exhibits much variety as regards its thick- 

 ness in different parts of the nervous sys- 

 tem. In the nerves it is more developed 

 than in the centres ; but even in the for- 

 mer it differs a good deal as to thick- 

 ness. We find it most developed in the 

 ordinary spinal nerves; in those of pure 

 sense it exists in small quantity. Both 



Nerve-tubes of the common eel . 



a. In water. The delicate line on its tllCSC elements of the tubular fibre 6 VI- 

 exterior indicates the tubular mem- 

 brane. The dark, double edged in- dcntlv afford mechanical protection to the 



ner one is the white substance of ^ x 



Schwann, slightly wrinkled. A. The substance which forms its aXIS; but doubt- 



same in cether. Several oil-globules 



have coalesced in the interior, and } ess O ne Or both of them maV have a further 



others have accumulated around the * 



exterior of the tube. The white sub- phvsioloffical office, in insulating the aXIS, 



stance has in part disappeared. Mag- * > . . . /, 



nined 300 diameters. a nd keepmgit distinct f rom any interference 



with constituents of neighbouring fibres. The chemical composi- 

 tion of the white substance, being obviously different from that of 

 the axis, sufficiently denotes a difference of function in these tw( 

 portions of the nerve-tube. The axis cylinder of the nerve-tul 

 though in general soft and pulpy, is in some instances of firm tex- 

 ture, and when broken projects beyond the w r hite substan* 

 (fig. 56, c). It then occasionally exhibits a well marked fibroi 

 character, and may even split into filaments. 



W'hen the tubes are quite fresh, and have been but little dis 

 turbed by manipulation, their form is that of a perfect cylinder. 

 Pressure, or separation, is apt to alter their shape by disturbing tl 

 position of the contained pulp, pushing more than is natural inl 

 some parts of the tube, and consequently diminishing the bulk oft! 

 contents in the adjacent parts ; so that the latter collapse, whi] 

 the former become distended, enlarged , and even varicose (fig. 52, 

 Nerve-tubes, that have been thus affected, sometimes present merely 

 a slight waviness of one or both margins, but more frequently a seric 

 of distinct swellings or varicosities separated by constricted portions 

 These swellings are found at very irregular distances from eacl 

 other, and vary extremely in shape and size. They are much moi 



