i64 A MANUAL Oh ANATOMY 



The mesodennic maxillary process gives rise to — (i) The chief part of the 

 superior maxilla, and the chief part of its palatal plate, the premaxilla and 

 anterior part of the palatal plate being developed from the mesodermic 

 investment of the globular process and its nasal lamina : (2) the corresponding 

 part of the upper lip; (3) the malar bone; (4) the palate bone ; and (5) the 

 internal pterygoid plate of the sphenoid. As has been stated, the lateral nasal 

 process gives rise to the ala nasi, and the nasal and lachrymal bones. 



From the inner surface of each maxillary process there grows, in a mesial 

 direction, a shelf-like plate of mesoderm. The two plates, right and left, are 

 known as the palatal shelves, and they meet along the median line. Each 

 gives rise to (i) the horizontal plate of the palate bone, and (2) the palatal 

 plate of the superior maxilla, except its anterior or premaxillary part, which is 

 developed from the nasal lamina of the globular process. In this manner the 

 hard palate is formed, and the primitive mouth, or stomodseum, is uow 

 divided into two chambers, upper and lower. The upper chamber represents 

 the two nasal fossae, which open anteriorly through the anterior nares, and 

 communicate posteriorly with the naso-pharynx through the posterior nares. 

 'J" he lower chamber constitutes the permanent mouth. The wide communica- 

 tion between the original nasal pits and the primitive mouth now becomes 

 considerably curtailed, and is only represented in adult life by the canals of 

 Stensen in the region of the anterior palatine fossa. 



Each maxillary process at its dorsal or proximal end — that is to say, where 

 it springs from the mandibular arch — contains for a short distance an off- 

 shoot from the cartilaginous bar of the mandibular arch, this prolongation 

 being known as the pterygo-palatine bar. The bar soon disappears, but its 

 mesodermic investment gives rise to (i) the internal pterygoid plate of the 

 sphenoid, and (2) the perpendicular plate of the palate bone. 



Mandibular Arches. — The two mandibular arches having parted with their 

 maxillary processes, meet each other at the median line, and form the caudal 

 or posterior boundary of the primitive mouth. The cartilaginous bar of each 

 arch is called Meckel's cartilage. The mesial portion of this cartilage gives 

 rise to the incisor part of the inferior maxilla or mandible. The chief part of 

 the body of the mandible, however, and the lower part of its ramus, as high 

 as the inferior dental foramen, are developed from the mesodermic invest- 

 ment of that portion of Meckel's cartilage which intervenes between the 

 incisor part and the level at which the cartilage gives off the pterygo-palatine 

 bar. This mesodermic investment also gives rise to one-half of the lower lip. 



From the foregoing description it will be evident that the first visceral 

 or mandibular arches build up the chief part of the facial portion of the 

 visceral-arch skeleton of the skull, as shown by the following summary: 



The maxillary process of each mandibular arch gives rise to (i) the chief 

 part of the superior maxilla, including the chief part of its palatal plate; 

 (2) the malar bone; and (3) the palate bone, including its horizontal plate. 

 The mandibular arch itself gives rise to the greater part of the mandible. 



The premaxillary portion of the superior maxilla, and the premaxillary 

 part of its palatal plate, are developed from the globular process of the mesial 

 nasal process and the nasal lamina of the globular process. The nasal 

 bone and the lachrymal bone are developed from the lateral nasal process of 

 the fronto-nasal process. 



Each inferior turbinate bone, as already stated, like the lateral mass of the 

 ethmoid (ethmo-turbinal), is developed in the lateral portion of the ethmoidal 

 division of the basal sheet of cartilage, or chondrocranium, from which it 

 becomes differentiated. 



The ethmoid bone, though tabulated as a cranial bone, is chiefly facial in 

 its disposition, inasmuch as it forms (i) a large part of the inner wall of the 

 orbit, (2) a large part of the outer wall of the nasal fossa, and (3) a large part 

 of the nasal septum. The only parts of the bone which belong to the base of the 

 granium are the cribriform plate and the crista galli. All parts of the bone are 

 developed from the ethmoidal division of the chondrocranium , as already stated. 



Tliere now only remain for consideration the vomer and the styloid process 

 of the temporal bone. 



