Under the most highly magnifying lenses of the microscope, three kinds of cells have been distinguished. The anterior gland is a collection of solid columns of cells, surrounded by blood spaces into which their secretion is undoubtedly directly poured. A gelatinous material, presumed to be the internal secretion of the gland, has, in fact, been observed emerging from the cells into the blood spaces. The posterior lobe, or gland, consists of secreting cells producing a glassy substance which finds its way into the spinal fluid that bathes the nervous system. The spinal fluid itself is a secretion of another gland at the base of the brain, the choroid. Nerves and internal secretion are associated here with a closeness symbolic of their general relations. From each portion of the gland (to stick to the accepted nomenclature of speaking of the two glands as one) an active substance has been isolated. Robertson, an American chemist, separated from the anterior lobe a substance soluble in the fat solvents, like ether and gasoline, which he christened tethelin. But P.E. Smith has shown that the active material is soluble neither in boiling water nor in boiling alcohol, the typical fat solvent. A number of facts favor the idea of the anterior lobe cells as stimulants of growth of bone and connecting and supporting tissues generally. From the posterior lobe, pituitrin, believed its internal secretion, has been obtained in solution. Pituitrin is a substance of many marvelous functions. In general, it controls the _tone_ of the tissues, of involuntary or smooth muscle fibres of the blood vessels and the contractile organs of the body like the intestines, the bladder and uterus. When injected, it will slowly raise the blood pressure and keep it raised for some time, and will increase the flow of urine from the kidneys and of milk from the breasts. It will also cause an intense continued contraction of the bladder and the uterus. It is also said to control the salt content of the blood upon which its electrical conductivity and other properties depend. Normally, there is a certain fixed ratio of the salts in the blood, which keeps them like the ratio in sea-water. Again, we have an example of the curious atavism of the internal secretions. The thyroid, remember, keeps the iodine concentration of the blood like that of the ocean, our original habitat. Pituitrin likewise does its part to maintain our internal environment as near as possible to what was once the surrounding medium. A substance somewhat similar has been found in the skin glands of toads.