The nervous system is the chief co-ordinating link between the various organs of the body, but in recent years it has been discovered that the relations of the different parts to one another are greatly influenced by substances known as internal secretions or ‘hormones’.  These substances are produced by ductless glands (the thyroid, suprarenals, &c.), from which they diffuse into the blood-stream and exercise a remarkable influence either on particular organs or systems, or on the body as a whole.  Some of these secretions act specifically on the involuntary muscles of the body, others control growth, others the development of the secondary sexual characters, such as the distinctive plumage of male birds, and also greatly influence the sexual instinct.  Much still remains to be discovered with regard to them, but it seems clear that they are of immense importance in the economy of the body.  It has been suggested, without much experimental support, however, that if a part of the body becomes modified by use or environment, it may produce a modified hormone, and that so, by the action of this on the germ-cells, the modification may be transmitted to subsequent generations.

Before leaving the subject of physiology in the more special or technical application of the term, reference must be made to another science the growth of which has been largely under the influence of medicine.  This is bacteriology, one of the newest branches of biology, and yet one which both from its practical importance and from the theoretical interest of its discoveries is rapidly taking a foremost place.  Of its practical achievements in connexion with disease, and with the part played by bacteria and other minute organisms in the life and affairs of man, it is not necessary to speak.  Every one knows the great advances that have been made in recent years in identifying (and to a less extent in controlling) disease-producing organisms, whether bacteria, protozoa (such as the organisms causing malaria, dysentery, etc.), or more highly organized parasites.  The attempt, however, to combat these pathogenic bacteria has led to discoveries of the highest importance with regard to the production of immunity, not only against specific germs, but against many organic poisons such as snake venom and various vegetable toxins.  That an attack of certain diseases leaves the patient immune to that disease for a longer or shorter time has of course been known for centuries, but it is a modern discovery that a specific poison induces the body to produce a specific antidote which neutralizes it, and the detailed working out of this principle and the study of the means by which the immunity is brought about promise to lead us a long way towards the central problem of the nature and activities of life itself.