CONSTRUCTION OF THE NERVES. 



329 



but are supported by a kind of framework formed by the dividing septa above 

 mentioned. Only after passing the ganglia do the nerves acquire a true funicu- 

 lated structure such as has been described for the peripheral nerves. The optio 

 nerve has throughout its whole course the structure of a nerve-root. 



The cerebro-spinal nerve-trunks and their branches always present a brilliant 

 whitish aspect, whereas the sympathetic nerves vary in appearance, some being 

 whiter, others grey or reddish in colour. The more grey-looking branches or bundles- 

 consist of a large number of the pale fibres mixed with a few of the medullated 

 kind ; the whiter cords, on the other hand, contain a proportionally large amount of 



Fig. 384. SECTIONS ACROSS PARTS OF THE ROOTS OF VARIOUS NERVES OF THE DOG, TO SHOW THK 



VARIATIONS IN SIZE OF THEIR CONSTITUENT F1BRF.S (Gaskell). 



The nerves were stained with osmic acid, and the sections are all drawn to one scale. 



A, from one of the upper roots of the spinal accessory. 



B, a rootlet of the hypoglossal. 



C, from the 1st cervical anterior root. 



D, from the 2nd thoracic anterior root. 



fine medullated fibres, and fewer of the grey ; and in some parts of the nerve grey 

 fasciculi and white fasciculi, respectively constituted as above described, run along- 

 side of each other in the same cords for a considerable space without mixing. This- 

 arrangement may be seen in some of the branches of communication with the spinal 

 nerves, in the trunk or cord which connects together the principal chain of sympa- 

 thetic ganglia, and in the primary branches proceeding thence to the viscera. 

 In the last-meutioned case the different fasciculi get more mixed as they advance, 



