208 



ON THE ELEMENTARY PARTS OF THE HUMAN FABRIC. 



dimensions. The fibres decrease in size as they approach the brain, either 

 directly, or through the medium of the spinal cord ; and in the brain itself they 

 continue to diminish, as they pass through the medullary towards the cortical 

 portion ; so that they are very commonly found of no more than 1 -7000th or 

 l-8000th of an inch in diameter, and sometimes as little as 1-14, 000th. Like 

 most other elementary structures, they are of considerably larger dimensions 

 in Reptiles and Fishes ; varying, according to Dr. Todd, from 1-1 260th to 

 l-2280th of an inch in the Frog; being in the Eel as much as the l-1040th 

 of an inch ; and in the optic nerve of the Cod, no less than l-650th of an inch 

 in .diameter.* 



244. Besides these proper tubular nerve-fibres, of which, in combination 

 with areolar and fibrous tissue, blood-vessels, &c., a large proportion of the 

 cerebro-spinal nerve-trunks are made up, there are certain other fibres, which 

 are peculiarly abundant in the trunks of the Sympathetic system, and which 

 are of different character from the preceding. They are chiefly distinguished 

 by their small size, their diameter not being above half or one-third of that of 

 the ordinary nervous tubuli. They are destitute of the double contour, which 

 has been shown to result in the preceding case from the presence of two dis- 

 tinct substances within the tubular investment; and their contents appear to 

 be homogeneous. And when they are aggregated in bundles, they possess a 

 yellowish-grey colour. Although these fine fibres exist in greater proportion 

 in the Sympathetic system than in the Cerebro-spinal, yet they are present 

 in great numbers in some of the nerves of the latter; and it is even question- 



Primitive fibres and ganglionic vesicles of human brain, after Piirkinje. A, ganglionic vesicles lying 

 amongst nerve-tubes and blood-vessels, in substance of optic thalamus; a, vesicle more enlarged; b, 

 vascular trunk. B, B, vesicles with variously-formed processes, from dark portion of cruscerebri. Mag- 

 nified 350 diameters. 



able, whether they may not be continuous with the ordinary tubular fibres. 

 They may be traced into the ganglia of the Sympathetic, into the ganglia on 

 the posterior roots of the Spinal nerves, and even to the ganglionic matter of 

 the Brain and Spinal Cord.f 



* Cyclopaedia of Anatomy and Physiology, Vol. HI., p. 593. 



f Much controversy li;is recently taken place in Germany, regarding the existence of a 

 set of fibres peculiar to the Sympathetic system. The grey or gelatinous fibres, described by 

 Remak, and (following him) by Al filler and others, :is essentially constituting the Organic 

 system of Nerves, are now generally admitted not to be entitled to the designation of nerve- 



