THE ENDOSKELETON: SKULL AND VISCERAL SKELETON 105 



generally forms in vertebrates a tough membrane, the sclera or sclerotic coat, which is the 

 outer coat of the eyeball. In some vertebrates, however, particularly birds and reptiles, a 

 ring of sclerotic bones arises in the sclera, but these always remain free from the skull. 



c) From the olfactory capsules: Additional cartilage bones, closely fused to and almost 

 indistinguishable from the ethmoid bones already mentioned may arise in the walls of the 

 olfactory capsules. These are the lateral ethmoids and the turbinals or conchae in part. 

 The latter are curiously scrolled or grooved bones situated on the walls of the nasal 

 cavities. 



3. Cartilage bones derived from the gill arches. 



a) From the pterygoquadrate cartilage: These cartilages form, as we have seen, the primi- 

 tive upper jaw and the dorsal half of the first or mandibular gill arch. The anterior part of 

 these cartilages may form the palatines, the middle part one or more paired pterygoids, while 

 the posterior part invariably gives rise to the paired quadrate bones to which the lower jaw 

 is generally articulated. It should be noted that the bones designated in various vertebrates 

 as palatines and pterygoids are in some cases cartilage bones derived from the pterygoquadrate 

 cartilage and in many cases are membrane bones. The quadrates are therefore the only 

 constant cartilage bones originating from the upper jaw (Fig. 37, p. 107). 



b) From Meckel's cartilage: These two cartilages constitute the two halves of the lower 

 jaw and the ventral half of the first gill arch. The greater part of Meckel's cartilages fails 

 to ossify, and either remains as a cartilaginous core of the lower jaw or disappears. The 

 posterior end of each cartilage commonly ossifies to form the articular bone which articulates 

 with the quadrate, forming a hinge joint for the lower jaw. The anterior tip of the cartilage 

 may rarely produce a cartilage bone (mentomeckelian bone). The articular bones are thus the 

 chief contribution of the lower jaw to the skull. 



c} From the second or hyoid gill arch: The dorsal portion of the hyoid arch forms the 

 hyomandibular cartilage as we saw in elasmobranchs. This ossifies into a hyomandibular 

 bone, which in many fishes suspends the lower jaw. The remainder of the hyoid arch together 

 with parts of the succeeding gill arches forms the hyoid apparatus. This is a plate or bar of 

 cartilage or bone, situated in the floor of the mouth cavity and throat, from which processes 

 extend posteriorly on each side to the otic region. It is variable in form and composition 

 in different vertebrates. 



d ) From the remaining gill arches: These arches ossify in fishes, but above fishes gradually 

 retrogress and become much modified, taking part in the formation of the hyoid apparatus 

 and transforming into the cartilages of the larynx. The larynx is the Adam's apple, so-called, 

 of man, and is the chamber at the top of the windpipe from which the voice comes. The 

 walls of this chamber are supported by cartilages which are the remains of the cartilages of 

 the gill arches. Wd, page 374, gives a figure to show the origin of the larynx of man from the 

 gill arches. 



4. Membrane bones added to the dorsal surface of the skull. As has been said, the 

 chondrocranium is open dorsally (except in fishes) and no cartilage bones are ever formed 

 there in any vertebrates, except at the extreme posterior end. Instead, the roof of the skull 

 consists of membrane bones, evolved from the dermal scales of the ganoid fishes. The chief 

 membrane bones covering the olfactory capsules and roof of the skull are: nasals, lacrimals, 

 prefrontals, frontals, post/rentals, and parietals. A number of others are present in fishes, 

 such as orbitals, temporals, operculars, etc., but as these do not persist, we shall not consider 

 them further. The earliest Amphibia and reptiles also had a considerably larger number of 

 membrane bones in the roof of the skull than do any living land vertebrates. These are 

 represented in Figure 3 64 and B. From such a generalized condition the dorsal aspect of the 

 skull of present-day land vertebrates has resulted through the loss of certain bones (see 

 Fig joCandP). 



