86 THE BODY AT WORK 



wide lymphatic vessels. The organ has a lavish supply of 

 blood. It is also well supplied with nerves. Colloid is the 

 secretion of the epithelial cells which line the globes. As these 

 globes have no openings, the secretion must be passed by 

 osmosis into the lymphatic vessels. There is abundant reason 

 for believing that by this route the products of the gland reach 

 the blood, and are distributed by the blood to all the tissues of 

 the body. And here it is important to notice that associated 

 with the thyroid gland are certain very small masses of tissue 

 termed " parathyroids." There may be four of these two on 

 the course of the large arteries which supply the thyroid gland 

 from above, two related with the almost equally large arteries 

 which supply it from below ; but the number varies. The para- 

 thyroids do not contain vesicles. They are solid masses of 

 epithelial cells, traversed by bloodvessels and lymphatics. 

 Yet, like the epithelial cells of the vesicles, they secrete colloid. 

 Granules of this substance are to be seen within their cells. 

 We cannot pass over the parathyroids without this reference, 

 since, small though they are, they seem to be quite as important 

 as the thyroid gland itself, judging from the effects which follow 

 their removal. 



In all vertebrate animals the thyroid gland has the characters 

 which we have described. What was it like in the ancestors of 

 the vertebrate races ? Its microscopic appearance in ver- 

 tebrates, the only animals in which we know it, is so anomalous 

 as to convince an histologist that it is a makeshift ; it looks 

 like an organ which, at a period no longer visible through the 

 mists of time, had a quite different function to perform. This 

 function it has lost some other organ has taken it on yet 

 it must do something which is useful to the organism. Other- 

 wise it would not have been preserved. It has been retained 

 for the sake of its by-function, for the sake of the internal 

 secretion which it produces. This is now the only work it has 

 to do. 



What was its prime function ? It is an axiom of biology 

 that an animal in its individual development recapitulates, 

 albeit with many omissions and abbreviations, the ancestral 

 history of its race. The thyroid gland appears in the embryo 

 as a diver ticulum of the anterior wall of the pharynx. It is 

 remarkable in being a single, median, unpaired diverticulum, 



