1881.] On the Minute Structure of the Thyroid Gland. 281 



(5.) Lymphatics and their Contents. — A network of lymphatics, 

 resembling, in its general characters, that previously described in the 

 dog, but differing apparently in the extent of its ramifications, has 

 been observed in some other mammals. The homogeneous or granular 

 contents of the lymphatics in the dog occur in both injected and in 

 uninjected glands, and in very variable quantity, the amount being 

 apparently independent of the sex or age of the animal. In the 

 thyroid gland of the pigeon no system of lymphatics could be injected 

 by the method of puncture after repeated attempts, but the injecting 

 fluid always entered the blood-vessels of the gland. The thyroid 

 gland of the tortoise contains a network of lymphatics of which the 

 smaller ramifications run between almost all individual vesicles. The 

 endothelium of these lymphatics presents well-marked areas of vary- 

 ing size and shape, stained darkly with nitrate of silver. In the 

 thyroid gland of the conger-eel no evidence of a system of lymphatics 

 was obtained by interstitial injection. 



(6.) Blood-vessels. — The arteries are ensheathed more or less com- 

 pletely in their accompanying lymphatics (dog, tortoise). The capil- 

 laries, on the contrary, run between the epithelium of the vesicles and 

 the smallest ramifications of the lymphatics (tortoise), leaving intervals 

 between them where the lymphatic is not separated from the epithelium 

 by blood-vessels. Projections of the capillaries between the epithelial 

 cells towards the cavity of the vesicles are frequently seen in the 

 tortoise. In the pigeon the intervesicular ramifications of the blood- 

 vessels are not so complicated nor so minute as in other animals (e.g., 

 tortoise, sheep), but resemble more the distribution of lymphatics in 

 some mammals. In birds also the veins frequently surround either 

 partially or entirely the arteries which they accompany. 



(7.) Undeveloped Portions. — Bodies consisting of portions of the 

 gland of which the development has not advanced beyond the stage of 

 convoluted " cylinders of cells," described by W. Miiller, are frequently 

 seen. They consist of a solid mass of more or less cylindrical rows of 

 cells, between which run septa of connective tissue and blood-vessels. 

 The " cylinders " are composed of cells resembling epithelial cells, 

 cubical or columnar in shape ; those on the surface of the cylinder, 

 next to the capillaries, being arranged at right angles to those vessels. 

 In the adult dog these undeveloped portions usually form distinct 

 bodies separated by layers of connective tissue from the rest of the 

 gland, and frequently lying in depressions on the surface of the organ. 

 In young animals, such as the kitten, they can be observed undergoing 

 a transformation into gland- vesicles. 



(8.) Thyroid Gland of the Frog. — In opposition to the statements of 

 most authors, but in general agreement with those of W. Miiller, 

 the thyroid gland of this animal has been found to consist of two 

 distinct bodies, situate one on either side, on the ventral surface of the 



y 2 



