As to the general nature of the influence, two chief theories have to be considered, viz., the nervous theory and the chemical theory. According to the former, we must assume that a stimulus originates in the reproductive glands, the testicles in the male, and the ovaries in the female, and that these glands excite a kind of reflex action--that is, that the stimulus passes to the central nervous system, and thence is "reflected" to the periphery, where it promotes, either the growth of particular parts of the body, _e.g._ the beard, or the development of definite properties in certain organs, _e.g._ the characteristics of the male larynx or of the female mamma. It is possible that the reflected impulse stimulates trophic nerves. But it may be that in cases of early castration the state of affairs is similar to that which obtains when from earliest infancy one of the sense organs is wanting, as a result of which the corresponding portions of the central nervous system are found to undergo atrophy.[48] On this assumption, the manifest arrest of the development of certain organs which results from castration is to be regarded as the sequel of a partial atrophy of certain portions of the brain. Of late, however, the chemical theory, that the results of castration are dependent on the lack of the internal secretion of the excised glands, has gained ground at the expense of the nervous theory. The reason for this change of view is that much which was unsuspected in former years has recently been learned about the chemical activities of other glands. It suffices to allude to the function of the thyroid body. According to this chemical theory, chemical substances are prepared in the reproductive glands, and these substances exert a specific influence in promoting the development of the secondary sexual characters. The same theory has been invoked to account for the alleged ill effects of sexual abstinence, it being suggested that the reabsorption of glandular products properly destined for excretion may give rise to toxic effects.[49] If it be assumed that the testicles can secrete substances upon the influence of which the development of the secondary sexual characters depends, it is obvious that these substances have nothing to do with the spermatozoa, inasmuch as the testicles exert the influence under consideration at an age at which the formation of spermatozoa has not yet begun. The substances that act in this way must be of a different kind. As was pointed out earlier in this book (p. 19), recent researches have shown that the testicles possess a twofold activity; and some French physicians even go so far as to say that the testicle is not a single gland, but two glands. They distinguish between the gland that prepares the spermatozoa and the interstitial gland.[50] Whilst the formation of spermatozoa subserves the generative act, the function of the interstitial gland is to prepare substances which pass into the