. Nutritional physiology . are in connection with the brain andresponsive to its changing conditions. A like relationshipcan be demonstrated for the glands that produce saUvaand for those which secrete sweat. Secretion and con-traction are two manifestations of metabolism which arealike regulated by the nervous system. In fact, if we setaside for the present the phenomena of consciousness, it isdoubtful whether we have any expression of the workingof the nerve-centers beside these two. What, then, is a gland? The word is used sometimesto designate a large organ like the liver, the pancreas, or
. Nutritional physiology . are in connection with the brain andresponsive to its changing conditions. A like relationshipcan be demonstrated for the glands that produce saUvaand for those which secrete sweat. Secretion and con-traction are two manifestations of metabolism which arealike regulated by the nervous system. In fact, if we setaside for the present the phenomena of consciousness, it isdoubtful whether we have any expression of the workingof the nerve-centers beside these two. What, then, is a gland? The word is used sometimesto designate a large organ like the liver, the pancreas, or akidney. Sometimes it is used with reference to a micro-scopic affair like an individual sweat-gland or one bf the THE WORK OF MUSCLES AND GLANDS 43 minute pits in the inner surface of the stomach or the in-testine. The fimdamental structure is the same in bothclasses. The microscopic gland is a depression of a cel-lular surface—a pocket, one might say—out of whichwhen it is active the secretion wells. The cells which. Pig. 5.—^The principle of glandular structure. In the upperfigure a simple microscopic gland is supposed to be laid open by asection along its vertical axis. The cells are seen to surround a recessinto which they discharge their secretion. Below, the same struc-ture is shown in its entirety, and in addition the encircling blood-vessels which contribute to make good the losses suffered by thesecreting cells. bovmd its cavity are the producers of the secretion, andare in turn dependent for renewal upon the lymph whichimderlies them and the blood which is flowing close superficial view would suggest that such a gland is afiltering device adapted for straining off certain portionsof the blood. This is, however, an entirely inadequate con- 44 NUTRITIONAL PHYSIOLOGY ception. Most secretions contain substances which werenot present in the blood at all, and which must, therefore,have been elaborated by the cells, of the gland. When weremember that the same bl
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