THE ANATOMY OF THE THYROID GLAND 207 



passing from the vasa lymphatica directly into the venae. The 

 relatively intimate relation between the veins and lymphatics 

 in fishes is well known; Wiedersheim ('07) and Favaro ('06) 

 have recently emphasized the fact so far as the tail vessels of 

 fishes were concerned. This intimate relationship seems to be 

 quite as obvious in the thyroid vessels and in those of the region 

 occupied by the thyroid sinus {vide supra). . 



That the vasa lymphatica are true lymphatics and not blood- 

 vessels is shown by the fact that in many cases they are of alto- 

 gether too small caliber to transmit the large red blood-cells of the 

 Elasmobranch fishes. Moreover, the quasi valvular nature of 

 their anastomosis with the veins, as already described, renders 

 highly improbable the regurgitation of blood-cells into the vasa 

 lymphatica even in the larger vessels. 



In mammalian thyroid glands, especially in dogs, one now and 

 then observes instances where the colloid has accumulated beyond 

 the bounds of the follicles, giving to sections of the organ the 

 appearance of a tissue completely infiltrated by the waxy colloid 

 substance. No such appearance was found in the Elasmobranch 

 thyroids which were studied, unless the follicles lined chiefly 

 by colloid cells could be so interpreted. 



SUMMARY 



1. The Elasmobranch thyroid gland closely simulates the 

 human, both in the form and structure of its follicles and the dis- 

 tribution of its blood-vessels. 



2. The gland rests upon the basi-hyal cartilage whose anterior 

 margin forms an excellent guide to its location. 



3. The pyramidal lobe of mammals is often represented in Elas- 

 mobranchs by a process passing forward and reaching the floor 

 of the pharynx through a notch in the anterior margin of the basi- 

 hyal cartilage; this notch is sometimes converted into a foramen. 



4. Baber's opinion that lymphatics are not present in the thy- 

 roid of Elasmobranch fishes was founded on insufficient evidence 

 and is incorrect. 



5. Lymphatics are present in considerable numbers both in and 



