208 THE DOG-FISH 



The second or hyoidean hoop is also large, and plays an 

 important part in slinging the jaWs to the skull. 



The third and succeeding hoops are more slender : they 

 surround the pharynx apd support the gills. 



To see the visceral skeleton properly, aroll of paper should 

 he passed in at the mouth-opening, and hack throtogh the 

 several hoops. 



1. The first or maxillo-mandibular arch is the stout hoop 

 of cartilage surroimding the entrance to the mouth, 

 and forming the jaws. It is divided by a joint about 

 the middle of each side ; the dorsal portion of each 

 side forming the upper jaw, and the ventral the lower 

 jaw : each of these is divided in the middle line in 

 front into right and left halves. 



a. The upper j aw consists of a pair of stout cartilaginous 



rods, wide behind and narrowing towards their 

 anterior ends, where they are united in the middle 

 line by ligament. They extend forwards as far as 

 the openings of the olfactory capsules, and are 

 connected by stout ethmo-palatine ligaments 

 with the base of the skull, at the antero-inferior 

 angles of the orbits. They bear teeth along the 

 ventral border of the anterior two-thirds of their 

 length. 



b. The lower jaw consists of a pair of wide flattened 



bars of cartilage, which are movably hinged with 

 the upper jaw behind, and bound together by a 

 median ligament in front. It lies, when the mouth 

 is shut, parallel to and immediately behind the 

 upper jaw. It bears teeth along the dorsal border 

 of the anterior two-thirds of its length on each 



2. The second or hyoidean arch is incomplete afeove : it 

 consists of a pair of rods the dorsal ends of which 

 articulate with the sides of the skuU, while the ventral 

 ends are connected together by a median plate of 



