THE ANATOMY OF THE THYROID GLAND 203 



of Raia than in the other species examined ; in Mustelus and Squa- 

 lus there is so Httle that one wonders at the relative compactness 

 of the organ. In these Selachii the epithelium rests directly upon 

 the walls of the blood-vessels, and they in turn consist of little 

 else than endothelium and form broad sinuses rather than capil- 

 laries or venules. 



The gland is inclosed by a very thin connective tissue capsule 

 and its tissue is thus always sharply defined from the surrounding 

 structures. In Mustelus and Squalus, and to some extent in the 

 other species, the broad vascular channels of the thyroid sinus 

 are in direct contact with the capsule, so that in sections the ven- 

 tral surface of the gland often appears clothed with an endothelial 

 coat derived from these vessels (figs. 16 and 17) ; a similar disposi- 

 tion of the vascular endothelium of collapsed blood-vessels is also 

 occasionally seen on the margins and dorsal surface of the gland. 



The blood vessels have been in each case carefully studied by 

 dissection, injection, sections, and transparent total mounts of 

 the gland. Both blood vessels and lymphatics were demonstrated 

 beyond doubt, though lymphatics have not hitherto been observed 

 in these fishes and their existence was denied by Baber ('81). 

 Fig. 9 shows the lymphatics filled with injection mass, lying 

 between the blood channels and the follicular epithelium; they 

 appear as perivascular lymphatics in the wall of the venae lym- 

 phaticae. Similar vessels, perivascular lymphatics, are found in 

 the walls of the arteries and veins of the thyroid, the thyroid sinus^ 

 and the arteries of the hypo-branchial system (fig. 8). 



The course of the larger blood-vessels was readily followed 

 in injected specimens in which the whole gland was examined 

 under the microscope. The arteries course upon the surface of 

 the gland, the major portion of them being always on the ventral 

 surface. Fig. 19 shows the distribution of the arteries in the thy- 

 roid of Mustelus, and figs. 3 and 4 indicate the relative area of the 

 gland supplied by the arteries of the right and left side, the left 

 thyroid artery, as in figs. 3 and 19 usually supplying the greater 

 part of the organ, though occasionally the major part, as in fig. 4, is 

 supplied from the right side. Twigs from the superfi3ial branches 

 here and there penetrate the gland, break into arterioles, and 



