24 PRESIDENT’S ADDRESS. 
sometimes too much gastric juice secreted; sometimes too tardy, some- 
times too rapid an absorption from the intestine; sometimes too little, 
sometimes too much blood pumped into the arteries, and so on. As 
the result of such lack of co-operation the life of the whole would cease 
to be normal and would eventually cease to be maintained. 
We have already seen what are the conditions which are favourable 
for the maintenance of life of the individual cell, no matter where 
situated. The principal condition is that it must be bathed by a nutrient 
fluid of suitable and constant composition. In higher animals this fluid 
is the lymph, which bathes the tissue elements and is itself constantly 
supplied with fresh nutriment and oxygen by the blood. Some tissue- 
cells are directly bathed by blood; and in invertebrates, in which there 
is no special system of lymph-vessels, all the tissues are thus nourished. 
All cells both take from and give to the blood, but not the same materials 
or to an equal extent. Some, such as the absorbing cells of the villi, 
almost exclusively give; others, such as the cells of the renal tubules, 
almost exclusively take. Nevertheless, the resultant of all the give and 
take throughout the body serves to maintain the composition of the 
blood constant under all cireumstances. In this way the first condition 
of the maintenance of the life of the aggregate is fulfilled by insuring 
that the life of the individual cells composing it is kept normal. 
The second essential condition for the maintenance of life of the cell- 
aggregate is the co-ordination of its parts and the due regulation of 
their activity, so that they may work together for the benefit of the 
whole. In the animal body this is effected in two ways: first, through 
the nervous system; and second, by the action of specific chemical 
substances which are formed in certain organs and carried by the 
blood to other parts of the body, the cells of which they excite to 
activity. These substances have received the general designation of 
‘hormones ’ (épudw, to stir up), a term introduced by Professor Starling. 
Their action, and indeed their very existence, has only been 
recognised of late years, although the part which they play in the 
physiology of animals appears to be only second in importance to that 
of the nervous system itself; indeed, maintenance of life may become 
impossible in the absence of certain of these hormones. 
Past slaved Before we consider the manner in which the nervous 
by the system serves to co-ordinate the life of the cell-aggregate, 
nervous tic 
pais ley let us see how it has become evolved. 
maintenance The first step in the process was taken when certain of 
—— the cells of the external layer became specially sensitive to 
Evolution of Stimuli from outside, whether caused by mechanical im- 
a nervous pressions (tactile and auditory stimuli) or impressions of 
an light and darkness (visual stimuli) or chemical impres- 
sions. The effects of such impressions were probably at first simply 
