VENOUS SYSTEM. 243 



size, can be traced forwards and inwards along 

 the outer wall of the pericaidial cavity; and 

 then forwards, in the floor of the moutli. 



Take especial care not to injure the branchial arteries 

 (p. 248). 



ii. The anterior cardinal sinus opens into the 

 Cuvierian sinus by a small aperture placed 

 immediately behind the fifth cerato-branchial 

 cartilage, and guarded by a very perfect valve 

 of which the ventral flap is much the larger. 



From this opening the sinus extends for- 

 wards as a wide irregular space, immedi- 

 ately above the gill-arches. In front of 

 the first gill this sinus communicates with 

 the orbit by the narrow pes orbital sinus. 

 This passes above the spiracle and below the 

 ear, and opens in front into he large orbital 

 sinus which" surrounds the eyeball and its 

 muscles, and is usually filled with blood-clots. 

 The orbital sinuses of the two sides com- 

 municate with each other by the inter-orbital 

 sinus, which runs across in the floor of the 

 skull, between the hinder parts of the orbits. 



Just in front of the first gill the anterior 

 cardinal sinus receives the large hyoidean 

 sinus, which lies in a groove on the outer 

 surface of the hyo-mandibular cartilage, and 

 may be followed downwards and inwards to 

 the floor of the mouth, where it communicates 

 with the inferior jugular sinus. 



Find the opening of the lejt anterior cardircal sinusinto the 

 Cuvierian sinus ; insert a seeker into it, turn the fish over, and 

 lay open the cardinal sinus along its whole length by slitting 

 through its dorsal wall ; wash out the blood-clots, and follow 

 it forwards to the orbit. Follow the hyoidean sinus, from the 

 anterior end of the cardinal sinus, downwards along the hyo- 

 mandibular cartilage to the floor of the mouth. 



b2 



