There is, however, another use which is subserved by the testicular products, a use which may indeed be said to be implied in those uses to which reference has already been made, but is yet historically the latest to be realized and studied. It was not until 1869 that Brown-Séquard first suggested that an important secretion was elaborated by the ductless glands and received into the circulation, but that suggestion proved to be epoch-making. If these glandular secretions are so valuable when administered as drugs to other persons, must they not be of far greater value when naturally secreted and poured out into the circulation in the living body? It is now generally believed, on the basis of a large and various body of evidence, that this is undoubtedly so. In a very crude form, indeed, this belief is by no means modern. In opposition to the old writers who were inclined to regard the semen as an excretion which it was beneficial to expel, there were other ancient authorities who argued that it was beneficial to retain it as being a vital fluid which, if reabsorbed, served to invigorate the body. The great physiologist, Haller, in the middle of the eighteenth century, came very near to the modern doctrine when he stated in his _Elements of Physiology_ that the sperm accumulated in the seminical vesicles is pumped back into the blood, and thus produces the beard and the hair together with the other surprising changes of puberty which are absent in the eunuch. The reabsorption of semen can scarcely be said to be a part of the modern physiological doctrine, but it is at least now generally held that the testes secrete substances which pass into the circulation and are of immense importance in the development of the organism. The experiments of Shattock and Seligmann indicate that the semen and its reabsorption in the seminal vesicles, or the nervous reactions produced by its presence, can have no part in the formation of secondary sexual characters. These investigators occluded the vas deferens in sheep by ligature, at an early age, rendering them later sterile though not impotent. The secondary sexual characters appeared as in ordinary sheep. Spermatogenesis, these inquirers conclude, may be the initial factor, but the results must be attributed to the elaboration by the testicles of an internal secretion and its absorption into the general circulation.[142] When animals are castrated there is enlargement of the ductless glands in the body, notably the thyroid and the suprarenal capsules.[143] It is evident, therefore, that the secretions of these ductless glands are in some degree compensatory to those of the testes. But this compensatory action is inadequate to produce any sexual development in the absence of the testes.