The vascular system no less than the alimentary is subject to both hormones and to nervous stimuli, inasmuch as nearly all its parts are innervated by the sympathetic system (for adrenaline acts on the nerve endings of that system). Thus Florey has shown in the intestine of the dog, that a sudden sound will cause the mucous membrane to pale in a matter of a few seconds. If the nerves to this piece of intestine be severed the noise will still cause the intestine to pale, but after a much longer time—in about a quarter of a minute. The paling is in each case due to constriction of the small vessels. In the first case the con striction is initiated by nervous stimuli reaching the mucosa along the branches of the splanchic supply; in the second the blanch ing is due presumably to adrenaline secretion. The case is not however entirely parallel to that of the nervous and chemical government of the pancreas inasmuch as the adrenaline secretion is itself due to nervous stimulation, so that the reduplication is merely a reduplication of the paths by which the nervous system can control the calibre of the vessels.
What applies to the blood vessels applies, so far as the relative roles of the nervous and the endocrine systems are concerned, to many other structures in the body which will at once occur to the reader. Many organs have not only a chemical and a nervous mechanism but also a double nervous supply. That however will be treated later in this article under Antagonistic Nerves.
Two organs in the mammals stand out as having no substi tutes, the lungs and the generative organs, and in each case the fact is a little interesting because lower down in the animal king dom that simplicity does not exist. Apart from the existence of gills, in the frog there is a cutaneous respiration so that it, even in air, has two alternatives. As regards reproduction of course the most primitive form is simply by fission. Reproduction by fission exists in many forms of life collaterally with sexual repro duction, but in all the higher types of the animal kingdom it has been given up.
The Principle of Antagonistic Nerves.—There are numer ous places in the body in which the actual condition of the organ at any moment is obtained by the balancing of antagonistic influ ences upon it. Four examples may be given.
(I) The rate of the heart beat is regulated by two nerves, one of which, the vagus, when stimulated, makes the heart go slower, while the other, the sympathetic, drives it faster. Normally its pace is neither so fast nor so slow as that of which it is capable, a fact to be accounted for by the balance of influences passing down the two nerves; if this balance alters the rate changes. (2) The diameter of the pupil of the eye is under the control of two nerves, one of which if stimulated dilates the pupil while the other constricts it; the actual size depends upon the relative influence of the two at any one moment. (3) The movements of the joints (say of the thigh joint) in walking are obtained by the balancing of the muscles which move the joint. It is not merely that alternately one set of muscles comes into action whilst the antagonistic set goes out of action : but each set is the whole time in a greater or less degree of tone. The whole action of the thigh
depends upon the relative degree of the various muscles which exert an antagonistic action. (4) In the highest animals delicate manual movements have been secured by the antagonism of the thumb to the fingers, especially to the first finger.
The four examples cited, if examined, show little similarity in the mechanism by which the antagonism is produced. In the first the nerves act presumably in the same structure in the heart tissue. In the pupil the two sets of nerves act on different muscles. These do not pull directly against one another like men in a tug of war; the muscle which tends to close the pupil is a ring, that which tends to open it is radial. In the thigh you have definite groups of muscles pitted against one another and pulling the bone in opposite directions, while in the finger and thumb there are actually different organs.
Thirty years ago thought was focussed on antagonistic nerves because they were supposed to initiate opposite phases of proto plasmic activity in the organs which they supplied. All living matter was conceived as capable of anabolism (building up) and katabolism (breaking down) ; the slowing of the heart due to vagus stimulation was supposedly the expression of accentuated anabolic activity, the quickening of the heart due to sympathetic stimulation was held to indicate increased katabolism. Of the known cases of antagonistic nerves there are so few in which the two sets of fibres end in the same tissue that their connection with anabolic and katabolic activity has passed out of the picture. It seems likely that their principal interest lies in the fact that by use of such antagonisms nature has built up a body capable of potentialities which it (the body) would not otherwise possess. By the balancing of the finger against the thumb more accurate and more delicate movements can be made than those of which either finger or thumb separately is capable. By the balancing of the thigh muscles against one another an accuracy of poise is secured which would not otherwise exist. The same is clearly true of the pupil, and had we the necessary discernment we could probably see the application of the general principle involved to the heart and blood vessels.
The Principle of Integrative Adaptation.—When the body is subjected to such adverse conditions that constancy of the composition of the blood cannot be maintained, the lack or excess of some one constituent may be of considerable gravity. In such circumstances some form of adaptation frequently takes place. This adaptation consists in an alteration in quite a number of factors each of which changes only a little though when the whole series of changes are integrated the net result is a very large degree of adaptation to the new environment. In such cases the body is not restored to its original degree of efficiency, but it has moved a considerable distance in that direction.