938 INFLUENCE OF DUCTLESS GLANDS ON METABOLISM. 
presumably reach the blood by means of the lymphatics or blood vessels 
of the organ, they have been termed " internal secretions." 1 
The Internal Secretion of the Thyroid Gland. 
The first internal secretion which may be considered is that of the 
thyroid gland. That the thyroid is a secreting gland no one who 
studies its structure and its mode of development can well doubt; 
except that it is unprovided in the adult state with a duct, it has all 
the features of structure of secreting glands. It is formed of alveoli 
which are lined by epithelial cells ; and although these cells have not 
been observed to exhibit changes characteristic of secretory activity 
so marked as those which have been noticed under like circumstances 
in the cells of ordinary glands, 2 we can observe the secreted material 
within the vesicles of the thyroid in the form of the substance known 
as " colloid." Various attempts have been made to isolate the active 
principle of the secretion ; these are referred to in a previous article. 3 
According to Drechsel, 4 there is probably more than one active substance, 
and the secretion may subserve more than one essential function. 5 
The gland is extremely vascular and very richly provided with 
nerves, and both blood vessels and nerves come into very close relation- 
ship with the secreting epithelium. The glandular structure of the 
thyroid is more obvious in young than in old animals, and as age 
advances, as has been shown by Hale White 6 and others, the organ 
undergoes a gradual process of degeneration, so that in advanced age its 
normal glandular structure can only with difficulty be recognised. 
Effects of ablation and disease. — As long ago as 1856, Schiff 7 found 
that extirpation of the thyroid in dogs is invariably followed by a fatal 
result. This observation, important as it now seems, fell for many 
years into oblivion, and it was not until clinical observations had again 
pointed to the importance of the gland that Schiff s experiments were 
remembered. It has indeed long been recognised that extensive 
disease of the thyroid, such as occurs in advanced forms of goitre, 
are accompanied by a swollen appearance of the integument, giving a 
misshapen aspect to the features and to the extremities, and leading to 
an idiotic or semi-idiotic condition, which is known as cretinism. In 
1873, G-ull 8 described a series of symptoms, and especially a condition 
of the integumental connective tissue, similar to that which is met 
with in cretins, the name " myxoedema " being subsequently given to 
it, because it was believed to be an cedematous condition charac- 
terised by the presence of a large amount of mucin. That there is an 
excess of mucin over that in ordinary connective tissue has been shown 
1 In one sense all the tissues and organs of the body form internal secretions, for they all 
pass into the blood materials which have been formed as products of their meta- 
bolism. 
2 Hiirthle, Arch. f. d. gcs. Physiol., Bonn, 1894, Bd. lvi. S. 1 ; Anderson, Arch. f. 
Anat. u. Enticcklngsgesch., Leipzig, 1894, S. 177 ; Schmidt, Arch, f. mikr. Anat., Bonn, 
1896, Bd. xlvii. S. 181 ; Galeotti, ibid., Bd. xlviii. S. 305. 
3 Halliburton, " Chemistry of the Tissues and Organs," p. 88. 
4 Ccntralbl. f. Physiol., Leipzig u. Wien, 1895, S. 705. 
5 Cf. also Xotkin, Virchow's Archiv, 1896, Suppl. Bd. cxliv. S. 224 ; Hutchison, 
Journ. Physiol., Cambridge and London, 1896, vol. xx. p. 474. 
6 Lancet, London, 1888, vol. i. p. 521. 
7 "Untersuch. ii. die Zuckcrbildung," Wiirzburg, 1859. 
8 Trans. Clin. Soc. London, October 24, 1873 (in vol. vii., 1874). 
