The importance of the internal secretion of the testicles in determining normal development has been recognized for a long time. Brown-Séquard especially emphasized the significance of testicular secretion on the general metabolism of the body. He claimed that the testicles secreted some substance that had a marvelous effect on the body, especially through the nervous system, in maintaining mental and physical vigor. He thought that the seminal fluid may be partly absorbed and taken into the blood, thus permeating the whole body and exerting a specific effect on the organism. Zoth and also Pregel1 in 1896 appear to have obtained definite evidence of the property of testicular extract to diminish muscular and nervous fatigue resulting from work, and also to lessen subjective fatigue sensations.
In 1840 Leydig discovered between the seminiferous tubules of the testicle groups of cells, which are ordinarily known as interstitial cells. These cells have become