184 JEREMIAH S. FERGUSON 



fluid injected into the thyroid or hyoid sinus can be forced back 

 into the venous channels of the thyroid j^land; an extreme pres- 

 sure will accomplish this result to a limited extent onl3\ I have 

 been able to find some traces of valves in microscopical sections. 



The hyoid sinus passes around the base of the hj^oidean hemi- 

 branch to connect with the jugular vein, through which a portion 

 of its blood is transmitted to the precaval sinus beneath the cora- 

 coid arch, and thence to the sinus venosus and auricle. The flow 

 through the hyoid sinus in this direction is quite intermittent, 

 and, as already indicated, it is chiefly dependent upon the muscu- 

 lar force of the pharynx as it alternately relaxes and contracts to 

 force water through the gill-openings. 



Blood is also transmitted from the hyoid sinus to the heart by 

 the more ventral and direct path through the mferior jugular 

 (anterior cardinal) vein. This vessel maintains a more constant 

 flow, receiving blood from the ventral cervical region and the 

 branchial arches, along the ventral ends of which it courses to 

 terminate in the precaval sinus. 



In Raia the thyroid sinus is thin, and its investment of connec- 

 tive tissue containing the lymphatic plexus is less pronounced than 

 in the other species studied, so that, except when the sinus is 

 fully distended with blood, the gland in Raia is not much obscured. 

 In Mustelus the sinus is larger and the fascia about it is more 

 voluminous so that the gland is usually more or less obscured, 

 though there is much individual variation: the same is true of 

 Squalus. In Carcharias the vascular walls in the sinus are so 

 thick, and the connective tissue about it so abundant that in most 

 of the animals examined the outline of the thyroid gland con- 

 tained within this mass could only be discerned on holding up 

 the stretched membranous mass between the eye and the bright 

 sun so that the intense transmitted light showed the yellowish 

 orange gland contained within the connective tissue mass. 



The thyroid lymphatic plexus forms an extensive group ot 

 vascular channels surrounding the gland and the vessels of the 

 blood sinus. It is contained in a fold of the deep cervical fascia 

 which stretches across from side to side between the ventral 

 ends of the first branchial clefts: it is broad in the mid-portion, 



