FUNCTION OF THE FIBRES. 401 



originally constituted a source of strength becomes a 

 source of weakness. Probably to this predominance of 

 fibres may be assigned the incapacity of acquiring new 

 ideas in old me. Intellectual vio^our is often mani- 

 fested by men of a very advanced age, but the vigour 

 is shown in dealino- with old trains of thouo;ht, not in 

 oriojinatino; new. To assume a new attitude of thouoht, 

 it may be necessary to develop new fibres ; and this can- 

 not be done in a tissue already too fibrous. A similar 

 hypothetical explanation suggests itself for the forma- 

 tion of fixed ideas, monomanias, habits, and tendencies. 

 But I will not venture fiu-ther into this hypothetical 

 region, the few anatomical facts hitherto ascertained 

 presenting too narrow a basis for such speculations. 

 One embryological indication may, however, be added. 

 The nerves of insects are, it is known, distinctly fibrous 

 (although in the bee and locust I have observed the 

 fibres occasionally melting into mere granules), but in 

 the larvae of insects the nerves are often mostly granu- 

 lar. Thus in the active predatory Dragonfly Larva — 

 the water-tiger, as it is called — I found the great ven- 

 tral chord possessing distinct fibres, but in many places 

 it was purely granular, the granules not having even a 

 linear disposition.* In the preparation I have made of 

 this object a very interesting analogy between the de- 

 velopment of nerve and muscle is presented. Muscles 



differentiation had as yet taken place between the grey and white 



matter ; and the Microscope, with a power of 750 linear, confirmed 



this impression, the structure of both grey and white matter being 



wholly vesicular and granular. 

 « 

 * In more advanced Larvae these chords are wholly fibrous. 



2c 



