686 JULIAN 8. HUXLEY 
remains of postural control were present in their patients— 
which indicated that some connexion was still present with 
pre-spinal centres—the mass-reflex did not appear. In other 
words, in the course of phylogenetic evolution, a compound 
mechanism has been evolved, the parts of which stand to each 
other in a relation of dominance and subordination. But here 
the dominance appears to be only slightly reversible, as opposed 
to the cases of Perophora and of mental regression. Here the 
subordinate system is so thoroughly under the control of the 
other (presumably owing to certain structural relations and to 
innate physico-chemical peculiarities inherent in synapses 
concerned with inhibition), that it is apparently impossible 
to tilt the balance so as to make the subordinate system the 
dominant one for long together, so long as both are in organic 
connexion. It is only when the two systems are separated 
from each other that the real nature of the subordinate system 
can be studied as it exists apart from controlling influence from 
without. As indicated above, differential inhibition through 
fear may induce a short temporary reversal of dominance.! 
Child has pointed out that a somewhat similar (and also 
simpler) relation subsists between the dominant and the 
subordinate regions in many low forms of animals, such for 
example as Planarians. Here, so long as the head region is 
exerting its dominant or controlling influence, other portions 
of the organism cannot form a head. But when this influence 
is removed, either by the amputation of the head or by the 
‘ physiological isolation’ of parts of the organism (by their 
removal, through growth, beyond the radius of influence of 
the head), then the most anterior part of the isolated region 
at once reacts by producing a head (Child, 1915), p. 96 et seq.). 
In Head’s spinal case, however, after isolation the subordinate 
system does not take on the characters of the dominant 
system, but assumes a form which is peculiar to itself. 
1 The views of Head and Riddoch have been recently criticized (e.g. 
‘Medical Science ’, vol. 4, 1921, pp. 141, 4380). The fact of decerebrate 
rigidity, however, would, among others, equally well serve to illustrate 
the principle of neurological dominance and subordination, although here 
we remain without phylogenetic analogies. 
