224 THE DOG-FISH 



of the first gill this sinus communicates -with 

 the orbit by the narrow post-orbital sinus. 

 This passes ahove 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, opposite the hinder part 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 forwairds to the orbit. Follow the hyoidean sinus, from the 

 anterior end of the cardinal sinus, downwa/rds along the hyo- 

 mandibula/r 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 dorgal wall being 

 firmly united to the body-wall, and its ventral 

 surface covered by the peritoneum. Its 

 anterior end forms a very large thin- walled 

 sac, three-quarters of an inch or more in 

 width. In front it lies at the side of the 

 oesophagus, and close to the hepatic sinus; 

 further back it lies dorsal to the oesophagus, 

 and alongside its fellow of the other side, with 

 which it communicates freely. Behind this 

 point, i.e. about two inches behind the peri- 



