STRUCTURE OF NERVE FIBRES. 157 



of the firm consistence of the sheath of Schwann surrounding 

 each fibre. The diameter of these non-medullated nerve fibres 

 varies very considerably. In the sympathetic they scarcely 

 exceed that of the medium-sized medullated fibres, but in the 

 olfactory nerves of many animals fibres may be found at least 

 three or four times thicker than the largest medullated fibres. 

 Such thick fibres are shown in fig. 22, a, taken from the nasal 

 fossa of a pike, consisting, when fresh, of a very soft, almost 

 fluid, finely granular mass, with parallel striae, contained 

 in a transparent and structureless sharply defined sheath, 

 in which, on the addition of acetic acid, nuclei make their 

 appearance. By carefully hardening the specimen the fibrillar 

 structure becomes very distinct, whilst at the same time the 

 whole contents of the sheath may be broken up into fibrillse of 

 the nature of primitive nerve fibrils, between which the fine 

 granules and molecules are interspersed to constitute an inter- 

 fibrillar mass. In Man and most other vertebrate animals the 

 fibres of the olfactory nerve are of less diameter than in fish, 

 and resemble rather those of the sympathetic nerve, except 

 that they are arranged in bundles within a common nucleated 

 sheath, so that funiculi are formed similar to those shown in the 

 subjoined fig. 6, taken from man. Here, as in the sympathetic 

 nerve c, the substance of the individual fibres is fibrillar, and 

 finely punctated, and probably consists of primitive fibrillse and 

 an inter fibrillar substance. 



According to the preceding account of the structure of the 

 nerve fibres, the following kinds may be distinguished : 

 Cl. Primitive fibrils. 

 ; 12. Fasciculi of primitive fibrils. 

 x i3. Primitive fibrils with medullary sheath. 



4. Fasciculi of primitive fibrils with medullary sheath. 



5. Fasciculi of primitive fibrils, invested by the sheath of 

 Schwann (as in the non-medullated nerve fibres of the sym- 

 pathetic, the olfactory nerve, and the nerves of the greater 

 number of invertebrate animals). 



6. Fasciculi of primitive fibrils, with medullary sheath and 

 the sheath of Schwann (as the fibres of most of the cerebro- 

 spinal nerves). 



1 and 2 may be distinguished as naked axis cylinders ; and 



o 2 



