THE SPINAL CORD AS A REFLEX CENTRE 



327 



THE WAY OUT 



Primitively the motor nerves also represent fibres passing from a col- 

 lection of ganglion-cells to the muscles of the corresponding body segment. 

 In the dorsal region this segmental arrangement of motor nerve fibres is 

 still traceable in the adult animal. In all other parts the morphological 

 has become subservient to a physiological arrangement. Every muscle of 

 the limbs contains elements from several segments, and is innervated there- 

 fore from several anterior spinal roots. Hence it follows that stimulation 

 of one anterior root produces no definite movement of a group of muscles, 

 but partial contraction of a number of muscles which do not normally con- 

 tract simultaneously. Thus stimulation of a sensory nerve may evoke 

 either flexion or extension of a limb, but not both simultaneously. Stimula- 

 tion of the motor roots will cause simultaneous contraction of both flexor and 

 extensor muscles. It is this subordination of morphological to physiological 

 arrangement in the limbs which has necessitated the formation of limb 

 plexuses. The nerve root is a mor- 

 phological collection of fibres ; the 

 nerve issuing from a limb plexus 

 and passing to a group of muscles is 

 a physiological collection. When it 

 is stimulated it evokes a contrac- 

 tion of a group of muscles which 

 are normally synergic, i.e. co- 

 operate in various movements. 



The fibres passing to the skeletal 

 muscles are large, about 14 yu, to 19 /A 

 in diameter, and their axis cylinders 

 represent the axons of 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 /u, to 3*6 //, in diameter (Fig. 

 163). 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 eommunicans 

 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 sug- 

 gested that cells of the lateral horn might be regarded as the origin of the 

 visceral nerve fibres. This suggestion has been confirmed by Anderson, 

 who has shown that section of the white rami communicantes brings about an 

 alteration in the cells of the lateral horn as a result of retrograde degeneration. 



Fio. 163. 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.) 



