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 the points of tangency of the 

 skeletal systems of the notochordal axis and the visceral axis. 



The diagram is a composite in that the visceral skeletal structures are shown 

 as they would appear in a fishlike vertebrate, while the ribs and sternum are 

 mammalian. 



For identification of visceral parts, compare with Fig. 120. 



