VENOUS SYSTEM. 251 



along the outer wall of the pericardial cavity ; 

 and then forwards, in the floor of the mouth. 



Talce especial care not to injure the branchial arteries (p. 255). 



ii. The anterior cardinal sinus opens into the 

 Cuvierian sinus by a small aperture placed 

 immediately behind the fifth cerato-bran- 

 chial cartilage, and guarded by a very per- 

 fect valve of which the ventral flap is much 

 the larger. 



From this opening the sinus extends for- 

 wards as a wide irregular space, immediately 

 above the gill-arches. In front of the first 

 gill this sinus communicates with the orbit 

 by the narrow post-orbital sinus. This passes 

 above the spiracle and below the ear, and 

 opens in front into the large orbital sinus 

 which surrounds the eyeball and its muscles, 

 and is usually filled with blood-clots. The 

 orbital sinuses of the two sides communicate 

 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 left anterior cardinal sinus into 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. 



