228 THE DOG-FISH 



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. 

 Prom this opening the sinus extends for- 

 wards as a wide irregular space, situated 

 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 commu- 

 nicate 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 ojpevdng 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 sirms, from the 

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

 mandibular cartilage to the floor of the mouth. 



iii. The posterior cardinal sinus extends back along 

 the dorsal surface of the abdominal cavity close 

 to the median plane, its dorsal wall being^ 

 firmly united to the body-waU, and its ventral 

 surface covered by the peritoneum. Its 



