72 



GENERAL ANATOMY. 



internodal segment contains an oval nucleus imbedded in the medullary sheath, 

 and occasionally more than one nucleus may be seen in the same internode. 

 Medullated nerve-fibres frequently present a beaded or varicose appearance : this 

 is due to manipulation and pressure causing the oily matter to collect into drops, 

 and in consequence of the extreme delicacy of the primitive sheath even slight 

 pressure will cause the transudation of the fatty matter, which collects as drops 

 of oil outside the membrane. This is, of course, promoted by the action of ether 

 (Fig. 49). 



The neurilemma or primitive sheath (sometimes called the tubular membrane or 

 sheath of Schwann) presents the appearance of a delicate, structureless membrane. 

 Here and there beneath it, and situated in depressions in the white matter of 



Schwann, are nuclei surrounded 

 by a small amount of protoplasm. 

 The nuclei are oval and somewhat 

 flattened, and bear a definite rela- 

 tion to the nodes of Ranvier ; one 

 nucleus generally lying in the 

 centre of each node, though in 

 some few instances two nuclei 

 may be found in the same node. 

 The sheath of Schwann, it is to be 

 noted, does not occur in the med- 

 ullated fibres contained within 

 the spinal cord and brain. 



Non-medullated Fibres. Most 

 of the nerves of the sympathetic 

 system, and some of the cerebro- 

 spinal (see especially the descrip- 

 tion of the olfactory nerve), con- 

 sist of another variety of nervous 

 fibres, which are called the gray 

 or gelatinous nerve-fibres fibres 

 of Memak (Fig. 50). These con- 

 sist of a bundle of finely striated 

 fibrillge enclosed in a sheath. 

 Nuclei may be detected at inter- 

 vals in each fibre, situated between the axis-cylinder and the neurilemma. In 

 external appearance the gelatinous nerves are semi-transparent and gray or yel- 

 lowish-gray. The individual fibres vary in size, generally averaging about half 

 the size of the medullated fibres ; but, on the one hand, the primitive fibrillae 

 formed by the breaking up of the cerebro-spinal fibres, as above mentioned, are 

 of hardly appreciable thickness ; while, on the other hand, some of the gelatinous 

 fibres (especially those on the olfactory bulb) are said to be three or four times 

 as thick as those of the cerebro-spinal nerves. 



Chemical Composition. The difference in the chemical composition of the white 

 and gray matter is indicated by the following analyses by Petrowsky of the brain 

 of the ox : 



Gray. White. 



Water 81.60$ 68.30$ 



Solids (percentage composition) : 



Proteids 55.37 24.72 



Lecithin 17.24 9.90 



Cholesterin and fat 18.68 51.91 



Cerebrin . 0.53 9.55 



Other organic compounds 6.71 3.34 



Salts 1.45 0.57 



FIG. 49. Magnified 300 diam- 

 eters, a. Nerve-fibre of the com- 

 mon eel in water. The delicate 

 line on its exterior indicates the 

 neurilemma. The dark double- 

 edged inner one is the white 

 matter of Schwann, slight- 

 ly wrinkled. 6. The same 

 in ether. Several oil-globules 

 have coalesced in the interior, 

 and others have accumulated 

 around the exterior of the tube. 

 The white matter has in part 

 disappeared. 



FIG. 50. A small ner- 

 vous branch from the sym- 

 pathetic of a mammal, a. 

 Two dark-bordered nerve- 

 tubes among a number of 

 Remak's fibres, b. 



The proteids in the above analysis practically represent the protoplasm, which 



