The self-improvement of the series of connecting tissues by exercise in other members, takes place in a more peculiar manner, rather mechanical than chemical and physiological. The motions of the joints are made easier by exercise; and making stiff joints movable is one of the most grateful objects of orthopedy. Herr Henke explains the unusual suppleness of the so-called India-rubber men as the result of relaxation of the ligaments, a disappearance of edge-surfaces of bone, and a diminished radius of curvature of the sliding surfaces, but particularly of a prolongation of the flesh-fibers at the expense of the tendons. Possibly an elastic tissue is formed in the ligaments of their limbs. Whether such a tissue grows in the vocal cords after exercise in vibrating them is still uncertain.
To make our statement complete, the increased ease in labor—bought, it is true, at the cost of greater danger of secondary bleedings—of those who have borne many times, belongs here. We may also regard as a self-improvement, although belonging to another region, the relaxing after-birth, and the accompanying reflex action of the breast upon the uterus.
The glands are another class of tissues the efficiency of winch is raised by exercise. The sexual glands—the milk-glands and testicles—are known to be capable of remaining at rest for years and even for life, while their tissues are subject to a considerable diminution, as is also normally the case with animals during the intervals between the periods of heat. Inversely the sexual glands attain a wonderful degree of production by means of alternations of rest and activity, as is exemplified by stallions, milch-cows, sheep, and goats. If the breast glands are not kept exercised by sucking, the udder by milking, they dry up and sink to rest till they are newly excited in sympathetic action with the uterus. The same can not be immediately proved of the fluids of secretory glands, but it is hardly doubtful that a digestive vessel that is kept active by two meals a day, with its glandular attachments, will dispose of a larger quantity of the various digestive fluids than that of a penitent. The kidneys of the practiced beer-drinker give passage to an incredible quantity of fluid. Finally, one who reads in the sketches of the manners of the last century of the continual weeping of the sentimental men and women of the time, will hardly be able to restrain the presumption that their tear-glands were brought up to the work by practice. We are as ignorant concerning the mechanism of the self-improvement of the glands through exercise as concerning the process of secretion in them. Since this process is different in nearly every gland, according to what nerves are introduced in one the secreted matter increases; in another becomes fatty; in another persists unchanged, but receives and gives out matter, or undergoes changes in itself—the problem appears twice as intricate and the information doubly scanty, so that at last we have to do again only with an increased accession of matter and more frequent innervation.