^8 ALLEN 



19. Directly laterad of the cephalic canal, in this region, there 

 is a gland-like body, the thymus, which is completely filled with 

 leucocytes, and in places one can see them transuding through 

 the walls of the lymphatic duct. 



As regards its mode of origin, position, and point of termina- 

 tion the cephalic sinus of Polyodon corresponds somewhat with 

 the second trunk described by Vogt in the salmon as emptying 

 into the common reservoir (i, pp. 137-8 and PI. L, Fig. i, 63). 

 Its position is also similar to Sappey's sinus curviligne in the 

 skate (25, p. 20, and PI. 5, Fig. i, 5). 



Hyo-ofcrctdar is sinus (Figs, i, 2, and 3, Hyo.O.S.). — The 

 sinus so designated is but little if any greater in diameter than 

 the cephalic trunk ; it is situated rather obliquely at the base of 

 the skull, and is distinctly a superficial duct, which at one point 

 crosses laterad of the jugular. Its length is apparently about 

 the same in a 25-pound specimen as in a 70-pound one. 

 Dorsad it receives a short vessel that collects the rich network 

 (Fig. I, L.N.{i)) from the skin and connective tissue in the 

 region of the cephalic trunk. Its mode of connection or termi- 

 nation in the cephalic trunk has already been described. In one 

 way or another it receives both forks of the_;fr5/ branchial lyni- 

 fhatic trunk (Figs, i, 2, and 3, Br.L.T.{i') and (i")), which 

 will be referred to in detail later on under the head of the 

 branchial l^-mphatics. Laterad and ventrad it receives, or vir- 

 tually is continuous with, the so-called hyo-ofercularis lymphatic 

 trunk, and, as shown in Fig. i, a valve is often present at the 

 junction of the two. 



Tracing this important canal (Figs, i, 2, 3, and 4, Hyo. 

 O.L. T.) backward, it was found immediately before making a 

 sharp curve caudad in order to pass along the outer surface of 

 the retractor hyomandibularis muscle, a little above the hj'o- 

 mandibular, that it was joined by a significant facial lymphatic 

 tf'unk (Figs. I, 2, and 3, F.L.T.). Throughout its entire 

 course the hyo-opercularis is distinctly a superficial canal, lying 

 directly beneath the skin ; it has its beginning from the hyoid 

 arch and the opercle, and collects a network (Fig. i, L.N.{'^)) 

 from the membranes and connective tissue of hyomandibular 

 and its retractor muscle ; this network communicates below 



