THE ANATOMY OF THE THYROID GLAND . 187 



of Favaro and I find the contents of the vesicles apparently 

 secreted into them as in the mammahan thyroid (fig. 10). The 

 larger vascular channels then are venae lymphaticae, readily in- 

 jected by puncture if the pressure is excessive, the veins being 

 easily entered because of their large caliber and extremely thin 

 walls; they transmit only lymph when the intravenous blood- 

 pressure is low within the gland, but fill with blood when from 

 any cause the pressure is raised. I have invariably found 

 some blood cells in the venae lymphaticae; I have never found 

 them filled with blood in all the three score animals I have 

 examined except in one case in which as a result of injury the thy- 

 roid gland was greatly congested. In this case they were filled to 

 distension. In microscopical sections I have been able to trace 

 the connection of the vasa with the venae lymphaticae (fig. 10). 

 I have been unable to demonstrate positively the presence of any 

 valves at the orifices of these vessels, but the extreme obhquity 

 of the anastomosis considered in conjunction with the very thin 

 vascular walls might well serve a valvular function when the blood 

 pressure is low, though with increased pressure and venous dis- 

 tention some blood would be forced back into the vasa lymphaticae 

 and even into the vesicles. The frequent occurrence of red blood 

 corpuscles within the vesicles of all animals is well known and in 

 the Elasmobranchs it is thus accounted for. The intimate rela- 

 tion between the venous and lymphatic systems pointed out by 

 Sabin ('09) would possibly suggest that an homologous vascular 

 relation may account for the presence of red blood corpuscles 

 within the vesicles of the mammahan thyroid gland. 



THE HISTOLOGY OF THE ELASMOBRANCH THYROID GLAND 



The thyroid gland in Elasmobranchs consists of a mass of ve- 

 sicular follicles (figs. 10, 11 and 13 to 18) which very closely re- 

 semble those of the mammahan gland. The vesicles are fined by 

 epithelium of a low columnar type, contain more or less colloid 

 material, and are loosely bound together by a connective tissue 

 framework which is very richly supphed with blood-vessels. 



The shape of the gland in Mustelus canis(fig 1,C) is sufficiently 



