4 THE PRESIDENTIAL ADDRESS. 



the growing rat, to quote Professor Harden, a quantity of vitamin A of 

 the order of 5^0 milhgram a day. Again, as regards sex determination, 

 the vahied discovery of a visible distinction betvi^een the nuclear threads 

 of male and female brings the further complexity that in such cases 

 sex extends throughout the whole body to every dividing cell. Again, 

 the association of hereditary unit-factors, such as body colour or shape 

 of wing, to visible details in the segmenting nucleus seemed to simplify 

 by epitomising. But further insight tends to trace the inherited unit 

 character not to the chromosome itself, but to balance of action between 

 the chromosome group. As with the atom in this heroic age of 

 physicists, the elementary unit assumed simple proves, under further 

 analysis, to be itself complex. Analysis opens a vista of further 

 analysis required. Knowledge of muscle contraction has, from the work 

 of Fletcher and Ploipkins on to Hill, Hartree, Meyerhof, and others, 

 advanced recently more than in many decades heretoifore. The engineer 

 would find it difficult to make a motive machine out of white of egg, 

 some dissolved salts, and thin membrane. Yet this practically is what 

 Nature has done in muscle, and obtained a machine of high 

 mechanical efficiency. Perhaps human ingenuity can learn from it. 

 One feature in the device is alternate development and removal of 

 acidity. The cycle of contraction and relaxation lies traced to the 

 production of lactic acid from glycogen and its neutralisation chiefly by 

 alkaline proteins ; and physically tO' an admirably direct transition from 

 chemical to mechanical effect. What new steps of mechanism all this 

 now opens ! To arrive at one goal is to start for others. 



But knowledge, while making for complexity, makes also for 

 simplification. There seems promise of simiDlification as to the 

 mechanism of reflex action. Reflex action with surprising nicety calls 

 into play just the appropriate muscles, and adjusts them in time and in 

 the suitable grading of their strength of pull. The moderating as well 

 as the driving of muscles is involved. Also the muscles have to pass from 

 the behest of one stimulus to that of another, even though the former 

 stimulus still persist. For these gradings, coadjustments, restraints, 

 and shifts various separate kinds of mechanism were assumed to exist 

 in the nerve-centres, although of the nature of such mechanisms little 

 could be said. Their processes were regarded as peculiar to the nerve- 

 centres and different from anything that the simple fibres of nerve- 

 trunks outside the centres can produce. We owe to Lucas and Adrian 

 the demonstration that without any nerve-centre whatever an excised 

 nerve-trunk with its muscle attached can be brought to yield, besides 

 conduction of nerve impulses, the extinction or attenuation or augmenta- 

 tion of them. That is remarkable, because the impulse is not gradable 

 by grading the strength of the stimulus. Any stimulus of strength 

 sufficient to excite the nerve-fibre at all, excites in it an impulse which 



