v SPINAL COED AND NEEVES 351 



XIII. We have seen that although the spinal cord is only the 

 instrument of the brain in the execution of voluntary movements, 

 it may exhibit activity independently of the higher centres in the 

 so-called reflex movements. 



We have further seen that the spinal reflexes vary not only 

 with the strength of stimulus and the excitability of the centres, 

 but also with the site of the stimulus. It is therefore necessary 

 to ascertain what part of the cord is concerned in individual 

 spinal reflexes, i.e. what is the localisation of their spinal centres. 



These problems are difficult to solve, and little progress has 

 yet been made in this direction. The spinal cord, as we have seen, 

 consists of a series of segments or myelomeres which are intimately 

 connected, and more or less linked together into a functional 

 solidarity, so that the different reflex centres cannot be distin- 

 guished by separating them apart from the shock this produces. 



It is certain that there are reflex centres which are more or 

 less scattered throughout the spinal axis, so that we cannot speak 

 of their localisation in any given region or segment of the cord. 

 Such are the spinal vasomotor centres and the centres for sweat 

 secretion discussed in Vol. I. p. 363 et seq., Vol. II. p. 495 et seq., 

 and the reflex centres which maintain a tonic and trophic influence 

 upon the muscles and the other tissues, as discussed in the present 

 chapter. 



In regard to the localisation of the motor (muscular) centres 

 in the cord, it was formerly supposed that there was a distinct 

 separation between them, according to their functions (i.e. specific 

 extensor centres, flexor centres, etc.). Eecent research has not, 

 however, confirmed this hypothesis. Lapinsky (1903) published 

 a series of experiments on dogs and rabbits with the object of 

 determining if there were definite centres in the cord for the 

 separate groups and segments of the musculature of the limbs. 

 He usually employed Gudden's method, and examined the retro- 

 grade degeneration which occurs in the nerve-cells after section of 



accurately the state of the various forms of sensibility. But as opportunities of 

 accurately correlating the clinical symptoms and the site of the lesion are rare, a 

 final conclusion can be reached only by such extensive investigations as can be 

 scarcely possible to any one clinician. On the other hand accurate clinical 

 observations on suitable cases, even when the site and extent of 'the lesion cannot 

 be verified, can at least show how the various components of sensation are grouped 

 and arranged in their passage through the cord. The later observations of many 

 writers, as Petren, Rothmann, and especially of Head and his collaborators, justify 

 the following conclusions : Pain and thermal sensibility are conducted through 

 the opposite ventrolateral columns ; two paths are open to tactile stimuli, one 

 in the homolateral dorsal column, the other in the opposite ventrolateral column 

 in the neighbourhood of the pain and thermal paths ; the faculty of localisation is 

 spatially associated with the tactile impressions in the cord ; the dorsal columns 

 convey uncrossed the impulses that subserve the sense of position and the appre- 

 ciation of movement, the recognition of size, shape, form, and weight, the appre- 

 ciation of vibration and the discrimination of simultaneous contacts (Weber's 

 compasses). 



