366 PHYSIOLOGY CHAP. 



The rami communicantes branch from a mixed spinal nerve, and 

 are themselves mixed nerves, containing both afferent and efferent 

 fibres. 



Gaskell (1886) was the first to make an exact study of the 

 structure, distribution, and function of the sympathetic nerve- 

 fibres. As in the dog the fibres that issue from the cells of the 

 spinal cord are myelinated, and as all the medullated fibres which 

 connect the cord with the ganglion chain run in the white rami, 

 he concluded that the majority of the fibres that pass from the 

 cord to the sympathetic must traverse the white rami. As he 

 further established that in the dog the white rami emerge 

 exclusively between the 2nd thoracic and 2nd lumbar roots, it 

 follows that the region of the cord between these segments is 

 the only part that gives origin to sympathetic fibres. 



Isolated stimulation of the white rami communicantes usually 

 presents insuperable difficulties, because they run along with the 

 grey fibres. In order to demonstrate the efferent fibres which 

 unite the cord and the sympathetic it is usual to stimulate the 

 entire spinal nerve above the point of exit of the rami communi- 

 cantes. The results of Claude Bernard, Langley, Sherrington, 

 etc., fully agree with Gaskell's conclusions. Only those spinal 

 nerves which give origin to white rami communicantes are capable, 

 on artificial stimulation, of exciting the organs innervated by the 

 sympathetic. The cervical nerves, which have no white rami, 

 are incapable of any such action. Bernard (1862) found nerve- 

 fibres able to dilate the pupil in the 1st thoracic, and not in the 

 8th cervical nerve; Sherrington (1892) observed the same on 

 the ape; and Langley (1897) found in the cat, rabbit, and dog 

 that the 1st thoracic is the highest nerve capable of a sym- 

 pathetic reaction on excitation. 



Analogous results were obtained from experiments on the 

 lower spinal nerves. Langley and Anderson (1895) obtained no 

 sympathetic reaction on stimulating the spinal nerves below the 

 lowest lumbar nerve that has a white ranius. In the dog this is 

 the 3rd or 4th, in the cat the 4th or 5th, in the rabbit the 5th, and 

 occasionally the 6th, in man probably the 2nd or 3rd lumbar nerve. 



The results obtained by the degeneration method agree fully 

 with the excitatory results. Section of those ventral roots that 

 give no sympathetic reaction on stimulation causes no degeneration 

 in the medullated nerve-fibres of the rami communicantes. This 

 holds, e.g. in Langley's demonstration on the cat (1896), for the 

 ventral roots of the 6th and 7th lumbar nerves, or the sacral and 

 coccygeal nerves. As Langley remarks, this is the more remark- 

 able seeing that in the cat the rami which apparently originate 

 in the 6th lumbar nerve may contain over 300 medullated nerve- 

 fibres. It follows that these fibres must originate in the higher 

 spinal nerves ; most of them, in fact, degenerate after transection 



