816 THE SPINAL CORD. 



intercostal space; if the excitation be increased in intensity, muscles 

 in the next space below and in the next space above likewise enter into 

 contraction. The relation of skin (centripetal) innervation to muscle 

 (centrifugal) innervation in this region is that the overlying skin is 

 innervated from segmental nerves of a segmental level rather farther 

 forward than the underlying muscle. 1 



If the isolated length of cord be brachial, the reflex movements 

 readily obtainable by stimuli applied to the limb itself are, in the 

 monkey, flexion-adduction of pollex, retraction at shoulder, extension at 

 wrist, flexion at elbow, flexion of fingers, and protraction at shoulder. 

 The hand, especially the palm, is a most efficient reflexigenous area. 



From an isolated length of the cervical cord of the cat comprised 

 between transections close above first cervical root and close below third 

 cervical, retraction of the pinna can frequently be obtained with striking 

 regularity and briskness, on excitation of its skin. 



The most striking difference between the condition of a body-region 

 with and without its spinal cord lies in the state of its skeletal 

 musculature ; in the former the muscles are not toneless 2 nor 

 altogether flaccid, nor do they atrophy beyond the effect of mere disuse. 

 As regards the visceral and vascular musculature, there is far less 

 difference between the two conditions. The visceral and vascular 

 reflexes are, on the whole, most easily evoked by stimuli applied to 

 reflexigenous regions that are themselves visceral or vascular, e.g. the 

 greater reflex effect on blood pressure on exciting a thoracic root than 

 on exciting merely its intercostal nerve. 3 Spinal reflexes playing upon 

 skeletal musculature are on the other hand less easily excited from the 

 viscera than from the skin and sense organs of joints and muscles. 

 The cutaneous stimuli which most potently evoke spinal reactions 

 employing skeletal muscles, seem without doubt " pathic." Probably it 

 is their " pathic " quality rather than their mere intensity as " pressure," 

 etc., which renders them peculiarly efficient. 



The functional powers exhibited by fragments of the length of the cord 

 furnish the basis of a theory which regards the spinal organ as a linked chain 

 of segmental centres. The morphological evidence of the segmentation of the 

 body into metameres, each with its segmental nerve, collaterally supports this 

 theory of its physiological segmentation. In lower forms the evidence of 

 such is clearer than in higher ; the theory is a good working hypothesis in 

 light of which to study the functions of even such complex spinal organs as 

 that of man. The spinal organ is certainly segmented in the sense that 

 fractions of its length possess considerable functional solidarity. It is also in 

 the same sense functionally divisible into lateral halves. The lurnbo-sacral 

 region of the cord can he split longitudinally with retention of the so-called 

 " deep reflexes," e.g. " knee-jerks," unimpaired on either side. 4 This recalls 

 the relatively high degree of independence of function attaching to each of the 

 pair of nerve cords in, for instance, the Arthropoda. 5 It is the ill-understood 

 phenomenon known as " shock " that especially tends to obscure the segmental 

 analysis of the spinal functions in higher animals. The fact that the referred 

 pains and areas of tenderness 6 accompanying certain visceral affections are 

 distributed over the skin surface in accordance with the scheme of distribution 



1 Sherrington, Phil. Trans., London, 1892. " Marshall Hall, 1839. 



3 J. R. Bradford, Joum. Physiol., Cambridge and London, 1889, vol. x. 



4 Sherrington, Proc. Roy. Soc. London, 1892, vol. lii. 



5 Bethe, Arch. f. d. gcs. Physiol., Bonn, 1897, Bd. lxviii. 



6 H. Head, Brain, London, 1893, vol. xvi., and 1894, vol. xvii. 



