THE SKELETON OF THE DOGFISH. VISCERAL SKELETON. 77 

 (2) THE VISCERAL SKELETON. 



The Visceral skeleton forms a series of seven cartilagi- 

 nous arches or hoops, surrounding the anterior part of the 

 alimentary canal, and enclosing a wide but rather shallow 

 space. 



(a) The first or mandibular arch is the largest of the 

 series, and forms the upper and lower jaws. Each half of the 

 upper jaw or palato-pterygo-quadrate bar is formed by a 

 thick cartilaginous rod which meets its fellow in the middle 

 line in front, the two being united by ligament. Each half is 

 connected to the cranium just in front of the orbit by the 

 ethnic-palatine ligament (fig. 6, 10), and at its hind end 

 articulates with one of the halves of the lower jaw. Each 

 half of the lower jaw or Meckel's cartilage (fig. 6, 12) is 

 a cartilaginous bar, wide behind but narrow in front, where 

 it is united to its fellow by a median ligament. Imbedded 

 in the tissue external to the upper jaw are a pair of 

 labial cartilages, and a similar but smaller pair are 

 imbedded in the tissue external to the lower jaw. 



The jaws are developed from a structure whose dorsal and 

 ventral portions subsequently become of very different import- 

 ance. The ventral portion forms both upper and lower jaws, 

 the former being developed as an outgrowth from the latter. 

 The dorsal portion forms only the prespiracular ligament 

 (fig. 6, 20), a strong fibrous band containing a nodule of 

 cartilage, and running from the anterior part of the auditory 

 capsule to the point where the jaws are connected with the 

 hyomandibular. 



(6) The hyoid arch consists of a pair of cartilaginous 

 rods which are attached at their dorsal ends to the cranium, 

 and are united ventrally by a broad median plate of cartilage, 

 the basi-hyal. Each rod is divided into a dorsal portion, 

 the hyomandibular and a ventral portion, the cerato-hyal. 



