2i 4 COMPARATIVE ANATOMY 



the vein which puts the orbital sinus in communication with 

 the anterior cardinal sinus. Immediately behind and below 

 this groove is a large foramen in the posterior wall of the 

 auditory capsule, through which the ninth cranial nerve passes 

 to the exterior, and f urther inwards, at the side of the foramen 

 magnum, is another large foramen for the exit of the tenth 

 cranial nerve. The flat floor of the cranium is marked just 

 behind the middle of its length by a pair of shallow grooves 

 running forwards and inwards from its sides. At the point 

 where they meet in the middle line, is an aperture through 

 which the internal carotid artery enters the skull. 



The jaws and suspensory apparatus of the jaws of the dogfish 

 are pretty firmly attached to the cranium by ligaments, but 

 they do not belong to the cranium but to the series of visceral 

 arches, the hinder members of which afford support to the 

 gill-pouches. The upper jaw of either side is a stout curved 

 bar of cartilage compressed laterally, with a prominent upper 

 ridge for the attachment of. the jaw muscles. Anteriorly it 

 meets its fellow of the opposite side in a median symphysis, 

 and it is attached to the cranium just behind the olfactory 

 capsule by a stout ethmo-palatine ligament. The posterior 

 extremity of the jaw stands well away from the cranium, and 

 bears on its lower surface an articular surface for the lower 

 jaw. The lower jaws are somewhat ^"-shaped, and laterally 

 compressed. They are connected together anteriorly by a 

 ligamentous symphysis, and posteriorly they articulate with 

 the hinder ends of the upper jaws. As the upper jaw is 

 held to represent the quadrate, pterygoid, and palatine bones 

 of higher vertebrates, it is commonly called the quadrato- 

 pterygo-palatine cartilage. The lower jaw may be taken to 

 be the equivalent of Meckel's cartilage in the frog and higher 

 vertebrates (vol. i. p. 28). The upper and lower jaws, taken 

 together, constitute the first visceral arch. Posteriorly they 

 are not directly attached to the cranium, but are suspended 

 from it by the upper members of the second visceral or 

 hyoid arch. The dorsal segment of the hyoid arch on each 

 side is a stout somewhat flattened bar of cartilage called the 

 hyomandibular, articulated by its proximal end with a concave 

 surface below the auditory capsule. It projects outwards, back- 

 wards, and downwards from the cranium, and its distal end 

 articulates opposite the angle of the jaw with the next segment, 



