218 STRUCTURE AND ENDOWMENTS OP ANIMAL TISSUES. 



brain ; and they diminish in the latter as they approach the cortical 

 substance. The fibres of the nerves of special sense are smaller than 

 the average, in every part of their course. 



375. The gelatinous fibres cannot be shown to consist of the same 

 variety of parts as the preceding ; no tubular envelope can be distin- 

 guished ; and the white substance of Schwann seems wanting. They 

 are flattened, soft and homogeneous in their appearance, bearing a con- 

 siderable resemblance to the unstriped Muscular fibres ; and, like them, 

 they contain numerous cell-nuclei, which are arranged with tolerable 

 regularity. These nuclei are brought into view by acetic acid, which 

 dissolves the rest of the fibre, leaving them unchanged. The gelatinous 

 fibres are usually of smaller size than the tubular, their diameter ave- 

 raging between the l-6000th and the l-4000th of an inch ; and they 

 sometimes show a disposition to split into very delicate fibrillae. Being 

 of a yellowish-gray colour, they have been sometimes distinguished as 

 the gray fibres. These two classes of fibres have been supposed to be 

 essentially distinct in character and office; the "tubular" having been 

 regarded as ministering to the Animal functions of sensation and 

 motion ; and the "gelatinous" as connected with the Organic or nutri- 

 tive operations. The facts which will be presently stated ( 388) 

 regarding their origin, however, as well as their joint existence in almost 

 every nerve, are decidedly adverse to this view ; and we shall find rea- 

 son to consider them as differing chiefly in grade of development. In 

 fact it appears that the very same fibre may be "tubular" in one part 

 of its course, and "gelatinous" in another. 



376. The Nerve-fibres appear to run continuously, from one extremity 

 of a nervous cord to the other, without anything like union or anasto- 

 mosis ; each ultimate fibre probably having its distinct office, which it 

 cannot share with another. The fasciculi, or bundles of fibres, however, 

 occasionally intermix and exchange fibres with each other ; and this 

 interchange may take place among either the fasciculi of the same trunk, 

 or among those of different trunks. Its object is evidently to diffuse 

 among the different branches the endowments of a particular set of 

 fibres. Thus we shall hereafter see that, in all the Spinal Nerves of 

 Vertebrata, one set of roots ministers to sensation, and another to 

 motion ; the sensory fibres are principally distributed to the skin, and 

 the motor fibres to the muscles ; but every branch contains both sensory 

 and motor fibres, which are brought together by the interlacement of 

 those connected with both sets of roots. In .the head, we have some 

 nervous trunks which have sensory roots alone ; and others which have 

 motor roots only ; these in like manner acquire each other's functions 

 in some degree by an interchange of filaments, the sensory trunk 

 receiving motor fibres, and the motor trunk receiving sensory fibres. 

 An interchange of this kind, upon a very extensive scale, takes place 

 between the Cerebro-spinal system, whose ganglionic centres are the 

 brain and spinal cord, and the Sympathetic system, whose centres con- 

 sist of a number of scattered ganglia. The former sends a large number 

 of fibres into the latter, by the twigs of communication near the origins 

 of the Spinal nerves, as well as by their connecting branches ; whilst 



