Physiological investigation does not support the existence of any strong affinity between old age and affection of the thyroid gland. It is known that removal of the thyroid is followed by cachexia only in young subjects, MM. Bourneville and Bricon[24] having shown that the tendency to cachexia after extirpation of the thyroid ceases almost abruptly at the age of thirty. That age may be taken as the limit of youth, of the time when growth is vigorous and the function of the thyroid most active. Cases of cachexia, where the thyroid gland has been removed in old persons from fifty to seventy, are very rare. Rodents (rats, rabbits) support the removal of the thyroid extremely well, without signs of cachexia, although these are normally short-lived creatures. According to Horsley[25] extirpation of the thyroid is not followed by cachexia in birds or rodents and is followed by it only very slowly in ruminants and horses; it produces the condition invariably but slightly in man and monkeys and extremely seriously in carnivora. If this series be compared with the information given in the next section of this volume on the relative ages which the animals in question attain, it will be seen that there is no correspondence. In short, whilst I do not deny that the vascular glands may take a share in the causation of senility, in so far as they are destroyers of poisons, I cannot agree with the theory of Dr. Lorand. [Illustration: FIG. 11.—Testis tissue from a dog aged twenty-two years. (From a preparation made by Dr. Weinberg.)]