President's address / by Henry H. Donaldson.

Henry Herbert Donaldson

Date:: 1911

Credit: President's address / by Henry H. Donaldson. Source: Wellcome Collection.

Provider: This material has been provided by The Royal College of Surgeons of England. The original may be consulted at The Royal College of Surgeons of England.

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This last fact becomes more plain when it is remembered that the medullary sheath of a medullated fiber has approximately the same volume as the axis which it surrounds (Donaldson and Hoke, ’05). Thus, for Example, the human brain beginning at birth with very little medullation of its fibers, acquires at maturity very complete medullation. To do this it must enlarge, and the en- largement is in great part accounted for by the formation of the medullary sheaths. Bringing together the foregoing observations, we may look upon the human brain as composed of about ten per cent, of sup- porting tissues, blood vessels and blood combined, and 90 per cent, of neurones. Two per cent, of the entire volume of the brain is in turn represented by the cell bodies of these neurones, the remaining mass, or 88 per cent., is nerve fibers, and so far as they are medullated, one half the volume of the fibers is to be assigned to the sheath and the other half to the axone. Thus a little less than one half the weight of the entire brain is com- posed of its functional neurones. In these respects the rat is very similar to man. Such then are some of the broad features of the structure of the brain, and such the argument for the suitability of the rat as an animal in which to study its growth. * So far we have dealt with growth changes under the usual or normal conditions. We turn now to our attempts to modify growth by altering some of the surrounding circumstances. The influence of exercise comes first. Ordinarily the albino rat does not get all the exercise it can take or that it should have. To meet this difficulty, some rats have been reared in revolving cages in which they take exercise, running often many miles a night as the automatic records show (Slonaker, ’07}. Such rats have brains which are thereby slightly improved by 2-3 per cent, as regards their weight, while the percentage of water in them is unmodified (Donaldson, Ti). When it is ap- preciated that this represents the first attempt we have made to test the effects of exercise, and that it was necessarily applied to animals in which the brain was already two-thirds grown, it will be seen that we have something to expect from further experi- ments along this line. Nevertheless, this is the only condition

President's address / by Henry H. Donaldson.

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