124 ALLEN 



the rear. Ventrad the occipital sinus communicates with the 

 pericardial sinus ; then passing mesadof the latter it again joins 

 the pericardial sinus from below, after gathering in the pectoral 

 sinus. The course of the pericardial sinus is along the inner 

 and anterior margin of the clavicle, and shortly after receiving 

 the ventral communication of the occipital sinus it is usually 

 joined by a fork of the ventral lymphatic trunk. Occasionally, 

 however, the ventral trunk does not divide, and in that case it 

 empties directly into one or the other pericardial sinuses. Each 

 pericardial sinus then continues cephalad along the side of the 

 ventricle, immediately outside of the parietal layer of the peri- 

 cardium, and when the bulbus is reached, sends inward a large 

 communicating branch that terminates in the so-called dorsal 

 pericardial sinus. This sinus, which is situated directly above 

 the bulbus arteriosus, has one or two communications above 

 with the inferior jugular, each orifice being guarded by a pair 

 of semilunar valves opening into the vein. 



The subcutaneous vessels surrounding the branchial cavity in 

 Polyodon take on the form of canals rather than sinuses. Situ- 

 ated obliquely at the base of the skull is the hyo-opercularis 

 sinus, which is formed from the union of the hyo-opercularis 

 and facial trunks. The latter in the main follows the external 

 jugular and its facialis branch, which in the region of the ad- 

 ductor mandibulae muscles communicates through a lymphatic 

 network with the so-called anterior facial lymphatic trunk. 

 This vessel travels along the surface of these muscles and ex- 

 tends into the snout region. The hyo-opercularis sinus also 

 receives the first branchial trunk, and opens mesad into a very 

 large cephalic canal that runs along the branchial chamber 

 directly above the jugular. The cephalic canal receives a great 

 trunk from below, formed by the union of the second, third and 

 fourth branchial vessels. When the supraclavicle is reached 

 the cephalic canal tapers down into a sort of papilla preparatory 

 to emptying into the cephalic sinus, and at this point it receives 

 from below what is designated as the pericardial trunk. This 

 vessel corresponds only in position to the pericardial sinuses of 

 Lepisosteus and other fishes, and in no way is it suggestive of 

 a sinus. In fact no pericardial sinuses were found in the region 

 of the heart. 



