832 THE SPINAL CORD. 



Influence of the location of the stimulus. — To the reactions 

 elaborated by each sense probably in some measure there pertains, 

 among other sensual qualities, a quality of sensual space. Of the four 

 senses which pre-eminently furnish space-perception, one has its end- 

 organs in the skin, another in the musculo-articular structures, and 

 these are therefore in great measure spinal senses. It is not surprising, 

 then, that per se the locus of the stimulus is for a spinal reflex an im- 

 portant determinant of the character of the resulting movement. 



The rule of spatial proximity given above (under short spinal reflexes) 

 partly expresses the influence of this factor in the reaction. Much that was 

 mentioned regarding longer irradiation illustrates it further. But the import- 

 ance of the locus, although high when broadly taken, does not appear obvious as 

 attaching to small differences of location in a more or less homogeneous sensory 

 field. Within a single large but functionally related sensifacient area, such as, 

 for instance, the skin of any one limb, considerable areal variation of the 

 incidence of the stimulus fails to at all markedly alter the resultant reflex 

 movement. 1 Thus, under appropriate conditions, a similar flexion of the hind- 

 limb at hip, knee, and ankle is elicited from any small area taken in the skin 

 surface of the limb. The flexions, though similar, are, however, demonstrably 

 not identical. 2 In the spinal monkey, excitation of the outer edge of the planta, 

 while causing dorso-flexion at ankle, in doing so generally brings the peronei 

 into play more than is the case when the flexion is excited from the inner 

 edge of the planta ; then the tibialis anticus predominates, causing some 

 inversion. In the frog, excitation of the skin of the knee on the dorsal 

 and on the ventral aspect respectively, alike evoke flexion at hip, knee, and 

 ankle, but in the former case the foot is somewhat everted, in the latter some- 

 what inverted. 3 



This influence of the location of the stimulus upon the character of 

 the movement is well seen in the reflex reactions of the spinal frog. It 

 furnishes a large part of the direct evidence of the so-called " purposive " 

 character of spinal reflexes. The physiological study of reflexes must be 

 objective, and from this point of view it is preferable perhaps to adopt 

 the expression " local sign," although borrowing it from the psychologist, 

 to denote a quality in virtue of which the character of the resulting 

 movement is partly determined by the spatial position of the reflexi- 

 genous area whence initiated. In the spinal dog (cervical transection) 

 the hind-foot is with fair accuracy brought to scratch the spot of the 

 shoulder under irritation, although that spot lie behind the spinal 

 lesion. The centripetal impulses from the spot — although they can 

 yield no sensation — possess therefore in the above sense " local sign." 



" Local sign " is in this sense a quality attaching to spinal and low-level 

 reactions throughout an enormous range of different nervous systems. It 

 is exemplified in the reflex actions obtaining between the manubrium and 

 the nectocalyx of Sarsia, and even more strikingly in a naked-eye medusa, 

 called on account of its localising reflexes Tiaropsis indicans. 4 The manubrium 

 deflects itself towards the stimulated part of the nectocalyx ; its extremity 



1 This is expressed by that rule of spinal reflex action which states that there is uni- 

 formity of response despite spatial variety of provocation {ride supra, p. 821). 



2 If the rule of spatial proximity be considered, moreover, to interact with the previous 

 rule, an explanation is offered for many of those minor differences obtaining in the broadly 

 similar movements, there being a tendency for the muscles belonging to the immediate 

 spinal vicinity of the skin stimulated to respond in preponderant degree. 



3 Schloesser, Arch./. Physiol., Leipzig, 1880, S. 303. 



4 Romanes, Phil. Trans., London, 1879. 



