. The chordates. Chordata. Motor System—Muscles and Skeleton 137 the mouth and the mouth is a "visceral" cavity. The dermal bones of the jaw originate in the deep tissue of the lining of the mouth. The hind part of the floor of the embryonic chondrocranium develops around the anterior end of the notochord and actually encloses a part of the notochord. The floor of the primary cranium, therefore, definitely lies in the notochordal axis. Bones developed in the dorsal wall of the oral cavity are ventral to the notochordal axis. Teeth, carried by certain bones of the upper jaw, certainly
. The chordates. Chordata. Motor System—Muscles and Skeleton 137 the mouth and the mouth is a "visceral" cavity. The dermal bones of the jaw originate in the deep tissue of the lining of the mouth. The hind part of the floor of the embryonic chondrocranium develops around the anterior end of the notochord and actually encloses a part of the notochord. The floor of the primary cranium, therefore, definitely lies in the notochordal axis. Bones developed in the dorsal wall of the oral cavity are ventral to the notochordal axis. Teeth, carried by certain bones of the upper jaw, certainly have more to do with the food below them than with the brain above them. The skeleton of the visceral axis and that of the notochordal axis are merely tangent at their anterior ends (Fig. 130), the region of tangency being the roof of the mouth and the floor of the cranium, but this fact does not justify assigning the whole visceral skeleton to the notochordal axis. In fact, reason could be found for considering such bones as the vomers (prevomers) and parasphenoid to be visceral rather than cranial, for they originate in the wall of the mouth and may carry teeth. The skull, therefore, in terms of axes, is a duplex thing, being pierced by both the notochordal axis and the visceral axis. Ribs develop in the myoseptums. They are products of the body- wall. They are attached to the vertebrae, but structurally they belong to the lateral body-wall and their primary function is to strengthen it and provide attachment for certain of its muscles. They embrace the coelom. If they are "axial," it is in relation to the coelomic axis. It would seem most significant to describe the ribs as constituting the lateral parietal skeleton (parietal referring to the body-wall), for. Fig. 130. Skeletal axes. (N) Notochordal axis; (S) sternal axis; (V) visceral axis. The white arrows, one at the anterior end of the upper jaw and the other on the dorsal member of the hyoid arch, indicate
Size: 2495px × 1001px
Photo credit: © Library Book Collection / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1900, bookcollect, bookpublisherphiladelphiablakiston
