Whatever other deductions may be extracted from these experiments, they prove beyond a doubt the existence of an endocrine factor in the process of aging, as well as an arterial.  They also demonstrate that the internal secretion of the sex glands, well advertised as it has been as the Elixir of Youth that Ponce de Leon, and Brown-Sequard with so many others, pursued in vain, is not the whole story.  For if it was, the duration of the new youth should be another span of life, whereas in actuality it is only a fraction of that time.  This fact, together with a number of others, make clear that while the gonads may be the jeune premier of the drama, the vitality of the plot depends upon the other endocrines.  Since old age is an exhaustion, permanent and irreparable of all the members of the ductless gland directorate, the reason becomes clear for the temporary quality of the rejuvenation effected by the procedures of Steinach.

Practically, then, the question at once arises:  which of the glands in particular are involved?  There is first that ubiquitous agent in the system, the thyroid.  Chemical analysis of it has shown that the iodine content decreases with the age of the individual, and becomes specially low after forty.  It is after the menopause in women that myxedema, the disease of complete degeneration of the thyroid, and of the physical and mental faculties, is most frequent.  The thyroid of old people exhibits, in varying degrees, signs of a similar degeneration.  Thyroid feeding, properly controlled, will clear up certain of the deteriorations of mind and body observable in the aged.  The grossness of the features lessens, a number of the pains go, muscular endurance increases, memory and intelligence do not remind one so forcibly of the old dotard in his second childhood.  Of course the improvement at present achievable is only relative.  But in the prematurely aging, decay invading a half accomplished maturity, marvels have been achieved at times with feeding of the gland.

The pituitary, too, begins to retrogress after the period of maturity.  And an early retrogression means a short maturity.  In women, the onset of an obesity, and coincidently, of a lazy and dull morale, coincides with this declension of the pituitary powers.  All the glands of internal secretion, in fact, shrink and shrivel as old age advances.  Only, as in other relationships, the predominating endocrine stamps its signature more visibly upon the documents of decadence than the others.  Pituitary types, as said, get fat and slow, thyroidal become bulky and stupid or thin and sour, the adrenal dark, shrunken and forever tired of life.  So type emerges, even in all-around glandular deficiency.

The problem of rejuvenation is the problem of recharging, or replacing all of the glands of internal secretion, at least the most important, the thyroid, the pituitary and the adrenals, as well as the gonads.  Longevity is perhaps largely a matter of preventing, or postponing their wane.  Beside, there is the prophylaxis of bacterial infections, and their all embracing corrosions—­which, too, have an endocrine aspect.