The above condition is only found in fishes, in higher animals the visceral skeleton is greatly reduced and modified.
The first or maxillo-mandibular arch is divisible into a dorsal portion, the palato-pterygo-quadrate bar, which forms the primitive upper jaw and enters into very close relations with the cranium, and a ventral portion, Meckel's cartilage, which forms the primitive lower jaw. The cartilaginous rudiments of both these portions disappear to a greater or less extent and become partly ossified, partly replaced by or enveloped in membrane bone.
The posterior part of the palato-pterygo-quadrate bar becomes ossified to form the quadrate, the anterior part to form the palatine and pterygoid, or the two latter may be formed partially or entirely of periosteal bone, developed round the cartilaginous bar. Two pairs of important membrane bones, the premaxillae and maxillae form the anterior part of the upper jaw, and behind the maxilla lies another membrane bone, the jugal or malar, which is connected with the quadrate by a quadratojugal. The premaxillae have a large share in bounding the external nasal openings or anterior nares.
In lower vertebrates the nasal passage leads directly into the front part of the mouth cavity and opens by the posterior nares. In some higher vertebrates, such as mammals and crocodiles, processes arise from the premaxillae and palatines, and sometimes from the pterygoids, which meet their fellows in the middle line and form the palate, shutting off the nasal passage from the mouth cavity and causing the posterior nares to open far back.
The cartilage of the lower jaw is in all animals with ossified skeletons, except the Mammalia, partly replaced by cartilage bone forming the articular, partly overlain by a series of membrane bones the dentary, splenial, angular, supra-angular and coronoid. In many sharks large paired accessory cartilages occur at the sides of the jaws; and in a few reptiles and some Amphibia, such as the Frog, the ossified representative of the anterior of these structures occurs forming the mento-meckelian bone. In mammals the lower jaw includes but a single bone.
The quadrate in all animals with ossified skeletons, except the Mammalia, forms the suspensorium of the mandible or the skeletal link between the jaw and the cranium; in the Mammalia, however, the mandible articulates with the squamosal, while the quadrate is greatly reduced, and is now generally considered to be represented by the tympanic ring of the ear.
The second visceral or hyoid arch in fishes consists of two pieces of cartilage, a proximal piece the hyomandibular, and a distal[10] piece the cerato-hyal. The cerato-hyals of the two sides are commonly united by a median ventral plate, the basi-hyal. The hyoid arch bears gills on its posterior border, but its most important function in most fishes is to act as the suspensorium. In higher vertebrates the representative of the hyomandibular is much reduced in size, and comes into relation with the ear forming the auditory ossicles; the cerato-hyal looses its attachment to the hyomandibular and becomes directly attached to the cranium, forming a large part of the hyoid apparatus of most higher vertebrates.
Behind the hyoid arch come the branchial arches. They are best developed in fishes, in which they are commonly five in number and bear gills. Their ventral ends are united in pairs by median pieces, the copulae.
In higher vertebrates they become greatly reduced, and all except the first and second completely disappear. In the highest vertebrates, the mammals, the second has disappeared, but in birds and many reptiles it is comparatively well developed.
3. The Ribs and Sternum.