i INTERNAL PROTECTIVE SECRETIONS 11 



Livini (1900) showed by histological methods that the para- 

 thyroids are fundamentally composed of epithelium cells, which 

 are gland cells proper. These cells elaborate two different 

 substances ; one, the principal, appears in the form of granules 

 or masses varying in size, which stain an intense green like 

 colloid substances with Galeotti's method ; the other, in the 

 form of minute granules, stains bright red, like the chroniatin of 

 the nucleus. The parathyroid secretion is poured out into the 

 pericellular lymph spaces, and reaches the blood by way of the 

 lymphatics. 



In certain animals small nodules of adenoid tissue with the 

 structural character of the thymus gland (Fig. 3) are associated 

 with the parathyroids, which suggests a common embryological 

 origin. Livini, however, demonstrated that the cells of these 

 masses (known as the tliymic lobules') are epithelial cells for 

 internal secretion, rather than lymphoid cells. He found, in 

 fact, that they produce a substance which completely fills the 

 thymic lobule, and usually increases its size. These modifications 

 in the cell mass are attended by serious nuclear disturbances,, 

 which eventually lead to the dissolution of the cells. It is worth 

 noting that this secretory product gives the same reaction as the 

 principal product elaborated by the cells of the thyroid and 

 parathyroid glands. 



According to Prenant and Fusari, the external parathyroids 

 have a common origin with the thymus, the internal with the 

 lateral lobes of the thyroid. The mode in which this transfor- 

 mation of the structure and specific character of the epithelial 

 cells is effected is unknown. In any case, we must exclude 

 the idea (which in the abstract appears rational enough, and 

 which was propounded by Gley) that the parathyroid is merely 

 embryonic thyroid tissue, which in the course of its develop- 

 ment may be transformed into the latter. The thyroid and 

 parathyroid are two structures specifically distinct in character, 

 and they cannot be vicariously substituted for one another 

 (infra). 



That both thyroid and parathyroid are secreting glandular 

 organs, and that the colloidal substance collected in the vesicles is 

 destined to be absorbed from the inter follicular lymph channels, 

 has been established by the histological work of Biondi with 

 Heidenhain (1889), Langendorff (1889), and a long series of other 

 observers, among whom are Vassale and Brazza, and Galeotti, 

 in Italy. As early as 1839 King demonstrated on dead bodies 

 that it is possible by exerting a certain pressure on the thyroid to 

 express the colloidal content of the vesicles into the lymphatics 

 that issue from the gland. Kohlrausch (1853) and Baber (1876) 

 showed under the microscope the presence in the intervesicular 

 lymph channels of colloid substances similar to that contained 



