The generation after him witnessed the triumph of the cell as the recognized unit of structure of the tissues, the brick of the organs.  It was soon found that the cells of the more familiar glands, like the sweat or tear glands, resembled the cells of the more mysterious structures named the thyroid in the neck, or adrenal in the abdomen, of which the function was unknown.  What had hitherto prevented classification of the latter as glands was the fact that they possessed no visible pathways for the removal of their secretion.  So now they were set apart as the ductless glands, the glands without ducts, as contrasted with the glands normally equipped with ducts.  Since, too, they were observed to have an exceedingly rich supply of blood, the blood presented itself as the only conceivable mode of egress for the secretions packed within the cells.  So they were also called the blood or vascular glands.

The names which became most popular were those which represented a contrast of the glands with the ducts, conveying their secretion to the exterior, as the glands of external secretion and the glands without the ducts, the secretions of which were kept within the body, absorbed by the blood and lymph to be used by the other cells, as the glands of internal secretion.  How different these two classes of glands are may be realized by imagining the existence of great factories manufacturing food products, which would diffuse through their walls into the atmosphere, to be absorbed by our bodies.

There are certain terms for the glands of internal secretion which are used interchangeably.  They are spoken of often as the endocrine glands and as the hormone producing glands.  Endocrine is most convenient for it stands for both the gland and its secretion.  Hormone is employed a good deal in the literature of the subject.  But it applies specifically to the internal secretion, and not to the gland.

THE EXPERIMENTAL PIONEER

All this clarification of the concept of the glands of internal secretion occurred in the first quarter of the nineteenth century.  However, no inkling of their real importance to the body, of which quantitatively they form so insignificant a part, was apparently revealed to anyone.  Not even the most daring speculation or brilliant guess work in physiology engaged them as material.  Thus Henle, the great anatomist, calmly affirmed that these glands “have no influence on animal life:  they may be extirpated or they degenerate without sensation or motion suffering in the least.”  Johann Mueller, the most celebrated physiologist of his day and contemporary of Henle, wrote in 1844 and coolly stated, “The ductless glands are alike in one particular—­they either produce a different change in the blood which circulates through them or the lymph which they elaborate plays a special role in the formation of blood or