D.—ZOOLOGY. 119 
from the effects of its spell of work has become temporarily physiologically 
younger. The second consideration is that the age of the animal counts 
as an important factor in the final result. The removal of one kidney from 
an adult throws the entire excretory function on to the other, and thereby 
increases general susceptibility to disease or breakdown where previously 
only local susceptibility occurred. But in the case of a child the result is 
quite different. In this case, the remaining kidney develops its reserves, 
forms additional tubules and glomeruli, and ultimately attains a volume 
equal to that of the two original organs. It is thereby enabled to continue 
its action on the lines of partial activity, and to afford each of its functional 
units their periodic phases of activity and repose. I trust that I may be 
pardoned for taking a leaf or two out of the book of pathology forthe purpose 
of illustrating, not only the principle of control, but also the great benefits 
to biological sciences that will accrue by a fuller mutual recognition of the 
advances made by pathologists and zoologists. 
Nervous Control. 
Another outstanding example of the working of control in the organism 
is afforded by the progress of neurology, in which your own earlier nomin- 
ated President and the President of Section I for this meeting have taken 
such a prominent part. The brain of man is now regarded as a hierarchy 
developed for control. The existence of its members, their activity and 
degree of suppression or of dominance and subordination, as well as their 
intricate relations to the body and its environment, are matters of interest 
to all of us, and their consideration may fitly introduce the larger aspects of 
control with which I shall presently deal. The brain is, in fact, the highest 
expression of the activity of that co-ordinated system of metabolic 
gradients which integrate the physical basis of life into individual being. If 
Imay venture for a moment into these deep waters, it is rather to illustrate 
the existence of control than to expound that relation of the nerves to the 
gradient hypothesis upon which Professor Child has recently issued 
a special memoir (11). 
The well-known experiments performed on the arm of Dr. Head, and 
since repeated by others, revealed more fully than before the normally 
suppressed nature of the thalamic complex. The acute but uncritical 
sensations that he experienced during the return of sensibility—the proto- 
pathic form of sensation—represent in all probability an early stage in the © 
sensations of vertebrates, and one connected with the optic thalami as that 
primary group of centres in the stem to which all sensory impulses con- 
verge. The subsequent return of normal epicritic sensibility marked the 
relative suppression of the thalamus by the higher cortical centres in the 
neopallium. The experiment caused a release of suppressed function. The 
lower order of the hierarchy was, for a time, allowed to exercise something 
of that disorderly, acute, and uncritical sensibility which has been in part 
incorporated into, but largely suppressed by, the more critical and dominant 
centres of the cortex. In some such way, the control that civilisation exerts 
upon society is thrown off by its retrograde units who indulge in dis- 
orderly, acute, and uncritical actions until forcibly restrained from so doing 
by the higher powers. 
In connection with the subject of nervous control and the development 
of social life, I should like to draw attention to the social insects whose 
