v SPINAL CORD AND NKliVKS 323 



that their final path is common to all receptive points wheresoever 

 they lie in the body, so long as they have connection with the 

 effector organ in question. The terminal path may, to distinguish 

 it from iuternuncial common paths, be called tin- Jinal common 

 path. The motor nerve to a muscle is a collection of final common 

 paths." 1 



Given this special arrangement of the various elements which 

 constitute a reflex arc, a series of important theoretical conclusions 

 capable of explaining the phenomena of facilitation and inhibition, 

 as seen in the reciprocal action of the various reflexes, can be 

 deduced. It is obvious that when several receptors connected with 

 the same common path are simultaneously excited, their individual 

 effects must be either summated so as to reinforce, or neutralised 

 so as to inhibit, according as the reflexes which they separately 

 excite harmonise or are incompatible. Sherrington terms the 

 former " allied," the latter " antagonist," reflexes. He has demon- 

 strated a series of such reflexes on the dog, which were either 

 allied to, or inhibitory of, the " scratch " reflex. 



IX. It is a vexed question whether the spinal cord is 

 capable of automatic as well as reflex activity. The rhythmic 

 respiratory movements that may persist after dividing the bulb 

 are of a doubtful character (Vol. I. p. 502). The tone of the 

 sphincters and of the blood-vessels, which we discussed in the 

 physiology of digestion and circulation (Vol. II. Chaps, III. and VI., 

 Vol. I. Chap. X.), is probably due to the action of constant or 

 frequently repeated extrinsic stimuli. The tone of the common 

 skeletal muscles in the resting state, which undoubtedly depends 

 on spinal tonus, is again not automatic but reflex in character, as 

 conies out plainly from Brondgeest's experiments. If the sciatic 

 of one hind-limb be divided in a spinal frog suspended vertically, 

 the flexor muscles of that limb are relaxed, while those of the 

 opposite limb are slightly contracted. This shows that the tone of 

 the flexor muscles of the hind-limb prevails over those of the 

 extensor muscles after removal of the brain, and that this muscular 

 tone depends on spinal tonicity. If instead of cutting the sciatic 

 its posterior roots are divided (Cyon), the tone of the flexors also 

 disappears. This shows that the tone of the spinal centre is not 

 automatic but reflex, i.e. it depends on a continuous wave of 

 excitation which flows through the sensory fibres to the centre, 

 and thence back to the muscles. 



Chloroform and ether, like transection of the afferent roots, 

 abolish the tone of the spinal cord. 



The tonic influence of the afferent roots seems not to be 

 derived exclusively from the sensitive cutaneous surface, as Brond- 

 geest assumed. In fact it persists in the frog even when the whole 



1 Sherrington, The Intcyrative Action of the Nervous System, London, 1906. 

 pp. 115 and 116. 



