56 VERTEBRATE ZOOLOGY 



Each visceral arch is a paired structure consisting of a right 

 and a left side which meet in the midventral plane. Each side 

 is also made up of several segments, which fall approximately into 

 a dorsal and a ventral half. 



The visceral arches may be divided into two groups : an ante- 

 rior group, consisting of the first two arches, which, with certain 

 membrane bones, form the skeleton of the jaws, the tongue, and 

 the opercle; and a posterior group, consisting of the last five 

 arches, which support the gills and are called the gill arches. 



We shall first study the anterior group. The first visceral 

 arch is the mandibular arch. The dorsal and ventral halves of it, 

 together with certain membrane bones, form the skeleton of the 

 upper jaw, the roof of the mouth, and the lower jaw. The 

 second visceral arch is the hyoid arch; of this the dorsal half 

 enters into relation with the mandibular arch and becomes the sus- 

 pensorium of the jaws,— that is, the connecting link between them 

 and the cranium,— and the ventral half forms the support of the 

 tongue and bears the opercular bones and the branchiostegal rays. 



The Mandibular Arch. Identify the following bones forming 

 the lower jaw, the ventral half of this arch. At its proximal end 

 is the large articular bone, by means of which it articulates with 

 the quadrate bone of the upper jaw. This is a cartilage bone, being 

 an ossification of the posterior end of Meckel's cartilage. In the 

 elasmobranch Meckel's cartilages form the lower jaw. In the 

 teleost they are also present, forming the axis of the lower jaw, 

 their posterior ends only having ossified. The forward and mid- 

 dle surfaces of the lower jaw are formed on each side by the large 

 dentary bone, which bears teeth. This is a membrane bone, having 

 formed around Meckel's cartilage, which is visible in a groove 

 on its inner surface, in a fresh skull. A small additional mem- 

 brane bone, the angular, is present at the hinder end of the jaw. 



Identify the bones of the upper jaw. Forming the forward end 

 and the lateral margins of this jaw are two pairs of membrane 

 bones, the premaxillae and the maxillae. The former bear teeth and 

 meet each other in the middle line in front ; the latter are without 

 teeth, being a pair of flattened rods back of the premaxillae, with 

 which their anterior ends articulate. 



