DIRECTION OF IMPULSES IN REFLEX ACTIONS. 819 



forwards; both when "spinal" go forwards only and not backwards. 

 So also with AmpJiioxus and the eel, 1 frog and tortoise. 



The young puppy, after severance of the upper part of the medulla 

 oblongata, has been found, 2 on application of a moistened finger to the 

 mouth, to suck the finger, wrapping the tongue round it, and at the same 

 time pushing out the legs> as young animals do when sucking. The 

 anencephalous foetus sucks when put to the breast. 



The condition of the cat and dog after transection at the calamus 

 scriptorius is not very dissimilar from that of the spinal frog, respira- 

 tion being artificially maintained, and the animal's temperature kept from 

 getting low. The arterial blood pressure is low at first, but that seems 

 to have little immediate effect upon the reflex functions of the spinal 

 cord. As the first " shock " passes off, a number of reflexes become 

 patent, including combined movement of all four limbs, of such a 

 character as to suggest that it represents a portion of the mechanism 

 of progression. 



SPECIAL EEFLEXES. 



The cord may, in its relation to the sensifacient surface and the skeletal 

 musculature, be considered divisible into right and left lateral halves, each 

 subdivisible into regions of neck (cervical, including pinna), fore-limb 

 (brachial), trunk (thoracic), hind-limb (crural), and tail (caudal). A reflex 

 action in which the stimulus applied to a reflexigenous area in one of the 

 above regions evokes a reaction in the musculature of another of the regions, 

 is conveniently called a long spinal reflex. A reflex reaction in which, the 

 muscular reply occurs in the same region as the application of the stimulus, is 

 conveniently called a short spinal reflex. Short spinal reflexes are, as a rule, 

 more easily and regularly elicitable than are long spinal reflexes. It might 

 further be convenient to allocate hard-and-fast boundaries to these regions, but 

 such limits would of necessity be artificial and arbitrary. The scope of the delimi- 

 tation is indicated and its purpose better served by comparing with one retina, 

 say of the bird, the one lateral half of the skin ; and with the corresponding optic 

 nerve, the corresponding lateral half of the spinal cord and bulb. Between these 

 comparable surfaces a difference exists, in that the reflexigenous field of the skin, 

 unlike the retinal, has instead of the one focal region of concentrated sensitivity, 

 a couple at least of such foci, namely, the relatively highly sentient skin at the 

 apex of each limb. But as the retina has muscles at call, so also the skin. 

 The closeness of nexus between a retinal point and the visual musculature is 

 graduate in degree, e.g. most close for the muscles of its own bulbus, next for 

 those of the contralateral, then for the neck muscles, etc. Similarly there are 

 degrees of nexal closeness between a point of skin and the related musculature; 

 its connection is most close with muscles of its own limb, next with those of 

 another limb or other region. The sole interest for physiology of the direc- 

 tion of nervous irradiation per se apart from light it may incidentally cast 

 upon the general physiology of nerve lies in its elucidation of the machinery 

 for working sentient surfaces. That the skin is a region which morphologic- 

 ally considered is composed of a segmental series, seems to have been allowed 

 greater weight in the estimation of its reflexigenous functions than is altogether 

 justified, at least in the higher vertebrata. That its segmental innervation 

 demonstrably limits existing reflex spinal functions in the mammal, is not 

 shown. 



The direction of spread of impulses in spinal reflex actions. 

 Eegarding short spinal reflexes, and the directions taken by the ex- 



1 Bickel, Arch.f. d. ges. Physiol., Bonn, 1897, Bd. Ixviii. 



2 Grainger, " Functions of the Spinal Cord," London, 1837. 



