CHAP, iv.] METABOLIC PROCESSES OF THE BODY. 797 



vessels derived from the superior and inferior thyroid arteries, the 

 branches of which, relatively large and frequently anastomosing, 

 end for the most part in capillary networks round the alveoli ; 

 from these capillaries and those of the septa the blood is gathered 

 into veins also relatively large, which forming plexuses on the 

 surface of the organ end in the superior middle and inferior thyroid 

 veins. The thyroid body is thus furnished with an abundant 

 supply of blood. 



The septa also contains besides a rich supply of lymph-spaces 

 a very large number of lymphatic vessels, which, both on the 

 surface of the organ and along the septa, are arranged in plexuses 

 of anastomosing trunks of considerable size. Small nodules of 

 adenoid tissue are also found in the septa. 



The nerves of the thyroid body are also abundant. They are, 

 in man, derived chiefly from the cervical sympathetic nerve, 

 namely, from the middle and lower cervical ganglia, but also from 

 the superior and inferior laryngeal nerves; their exact terminations 

 within the organ are not known. 



The ' accessory ' thyroid bodies often found are of the same 

 nature as the main body. 



The name ' colloid substance ' is usually given to the contents 

 of an alveolus when the consistency of these is marked ; but both 

 the mere glairiness of the contents of one alveolus and the almost 

 solid nature of those of another appear to be due to the relative 

 abundance of a constituent of the contents which, though it has a 

 superficial resemblance to mucin, is not true mucin ; indeed it 

 seems like the mucin of bile ( 244) to be of the nature of nucleo- 

 albumin. This mucin-like body, which itself is sometimes spoken 

 of as ' colloid substance,' the term being often used very vaguely, 

 appears to be secreted by the epithelium lining the alveoli. If in 

 an animal (dog) the greater part of the thyroid be removed, a 

 small portion only being left, the alveoli of this remnant seem to 

 take on great activity accompanied with considerable changes in 

 the cells. Many of the cells appear to become loaded with colloid 

 substance which they discharge, sometimes with rupture of the 

 cell, into the cavity of the alveolus. The colloid substance, which 

 may be stained with appropriate reagents, and which generally 

 fills the alveolus, is at times mixed with epithelium cells and 

 blood corpuscles. In some alveoli the epithelium appears to be 

 undergoing a sort of degeneration ; and a distinct extravasation of 

 blood into an alveolus is occasionally met with. 



There is a considerable amount of evidence that the colloid 

 substance may find its way out of the alveolus into the surrounding 

 lymph- spaces and so into the lymphatic vessels, and that not by a 

 rupture of the alveolar wall but by a passage of the material 

 between the epithelium cells ; the lymph-spaces and capillary blood 

 vessels lie close under the bases of the cells, the basement mem- 

 brane of the alveolus being often wanting or inconspicuous. 



