370 PHYSIOLOGY 



large nerve- cells in the anterior horn. In the dorsal region of the 

 cord in man, from the second dorsal to the second lumbar nerve- 

 roots, the anterior roots contain, besides these coarse fibres, a number 

 of fine fibres about 1-8 M to 3-6 //, in diameter (Fig. 164). These fine 

 fibres were shown by Gaskell to leave the nerve shortly after the 

 junction of the two roots, to pass as a white ramus communicans 

 to the sympathetic. Excitation of the white rami evokes various visceral 

 effects, such as dilatation of the pupils, augmentation of the heart, 



contraction of blood-vessels, 

 inhibition of the gut, erection 

 of hairs, &c. Gaskell pointed 

 out that the outflow of these 

 fine fibres coincided with the 

 existence of a prominent lateral 

 horn in the grey matter, and 

 suggested that cells of the 

 lateral horn might be regarded 

 as the. origin of the visceral 

 nerve fibres. This suggestion 

 has been confirmed by An- 

 derson, who has shown that 

 section of the white rami com- 

 municantes brings about an 

 alteration in the cells of the 

 lateral horn as a result of 

 retrograde degeneration. 



FIG. 164. Section across the second thoracic 

 ventral nerve-root of the dog (stained with 

 osmic acid) to show varying sizes of the con- 

 stituent fibres. (GASKELL.) 



CENTRAL PATHS OF SPINAL REFLEXES 



The impulse entering the cord is thus able to affect immediately 

 a number of systems of neurons, namely, cells in the anterior horn, 

 in the posterior horn, in Clarke's column, in the substantia gelatinosa, 

 in the lateral column of the same side of the cord, and the corre- 

 sponding groups of cells on the opposite side of the cord either directly 

 by crossing collaterals or indirectly through cells which send their 

 axons across the middle line. Through the ascending and descending- 

 fibres of the posterior columns it can also set into action the reflex 

 mechanisms of adjacent segments of the cord. In addition to this 

 direct spread of afferent impulse up and down the cord there is an 

 anatomical basis for a co-ordination between the grey matter of 

 different levels. This co-ordination is effected through the inter- 

 mediation of the internuncial or intra-spinal fibres which pass up 

 and down the cord from segment to segment. The course of the 

 descending fibres may be studied by carrying out a total transection 

 of the spinal cord at the sixth cervical vertebra, and six months 



