282 



AXIMAL PHYSIOLOGY. 



[LESSON 87. 



outer one is a delicate, structureless sheath, called the Neurolemma 

 Fir 8% (neuron, a nerve ; lemma, a sheath). 



1264. Within this sheath are the 

 nerve fibres, as they have been called, 

 which are really tubes containing a me- 

 dulla, or pulp. 



1265. These ultimate tubes possess a 

 structureless sheath of extreme delicacy; 

 they are found composing the nerves of 

 all descriptions ; belong as much to the 

 gray, or cortical, as to the white, or me- 

 dullary substance of the brain, of the 

 spinal chord (where it exists), and of the 

 ganglia. 



1266. The pulp, contained in the 

 tubes, known as the " white substance" of 

 Schawn, who first described it, is, when 

 quite fresh, perfectly homogeneous, fluid, 

 and viscid, like a thick oil, but every 

 method known of preserving it, coagulates 

 or otherwise alters it. For this purpose a 

 saline solution (eight ounces of salt, to one 

 quart of water) appears to be the best 

 adapted, as it changes it the least. 



1267. It has been remarked that it 

 readily dissolves in water (Mollusca), 

 leaving only a series of empty tubes, and 

 this is true even in the human subject. 

 Preparations of nerves, made for the mi- 

 croscope, are subject to pressure, and 

 under its influence the whole contents of 

 the nerve tubes (in time) become squeezed 

 out. 



1268. In addition to the white sub- 

 stance of Schawn, these tubes contain in 

 their centre a tube of the most delicate 

 transparency ; this is called the axis- 

 cylinder. 



1269. A figure of the axis-cylinder 

 obtained by pressure from a preparation of the median nerve (human), 

 is given in Fig. 397. The same preparation also displays the white 



Cerebio-spinal axis (brain, spinal 

 chord) of man. 



