i62 A MANUAL OF ANATOMY 



turbinate bone. Though known as the ethmoidal division or region, it 

 gives rise not only to the ethmoid, etc., but also to the presphenoid and 

 alisphenoids posteriorly, whilst anteriorly and mesially it dips down to form 

 the septal nasal cartilage, and is prolonged as the core of the fronto-nasal 

 process. 



Those bones of the cranium which are ossified from centres appearing in 

 the basal sheet of cartilage, or chondrocranium, are called endochondral or 

 cartilage bones, and the following is a summary of them : 



1. Occipital, except the interparietal part, or upper division of the tabular 

 portion. 



2. Sphenoid, except the two internal pterygoid plates, but including the two 

 sphenoidal turbinate bones. 



3. Ethmoid. 



4. Petro-Mastoid portion {greater part) of the temporal bone. 

 Membranous Cranium, or Tegmental. — As previously stated, the primitive 



membranous cranium undergoes chondrification along its basal part. The 

 tegmental part is not involved in this process, but forms a connective-tissue 

 membrane, which subsequently gives rise to certain cranial bones. From 

 the fact that these bones are ossified from centres which appear in this mem- 

 brane they are called membrane-bones, and, inasmuch as they constitute 

 the covering of the cranial vault, they are known as tegmental bones. The 

 following is a summary of these bones: 



1 . The Interparietal, or upper division of the tabular portion of the Occipital. 



2. The two Parietal bones. 



3. The Frontal bone (originally in two halves). 



4. The Squamo-Zygomatic portion of each Temporal bone, as well as the 

 pars tympanic a. 



The Visceral Arch Skeleton of the Skull. 



The visceral arch skeleton comprises the face and the hyoid bone. 



The Face. — The structures concerned in the development of the bones <5f 

 the face are — (i) The fronto-nasal process, and the ethmoidal division of the 

 basal .sheet of cartilage which constitutes the chondro-cranium ; and (2) the 

 first visceral, or mandibular, arch, right and left, along with the maxillary 

 process of each of these arches. 



Fronto-Nasal Process. — This process occupies a median position, and con- 

 sists of a central part or core, and an external covering. The core is carti- 

 laginous, and is a downward and forward extension of the anterior part of 

 the ethmoidal division of the basal sheet of cartilage which forms the chon- 

 drocranium. The external covering of the process is derived from the primi- 

 tive cerebral capsule on the ventral aspect of the fore-brain, and is therefore 

 membranous, being formed by mesenchyme. 



The process grows downwards and forwards, and from either side of its 

 base a bud grows in a downward and forward direction, this bud having 

 the same structure as the parent process. 



The fronto-nasal process then becomes somewhat complicated. The central 

 portion of it is called the mesial nasal process, and this terminates below in 

 two well-marked protuberances, one on either side of the median line, called 

 the globular processes. The lateral bud on either side becomes the lateral 

 nasal process. The fronto-nasal process, then, consists of (i) an undivided 

 upper portion, (2) a mesial nasal process, terminating in two globular pro- 

 cesses, and (3) two lateral nasal processes, right and left. As seen super- 

 ficially on the embryonic face, the mesial nasal process and its two globular 

 processes lie between the widely-separated ventral or mesial extremities of 

 the two maxillar processes, and form the cephalic, or superior, boundary of 

 the primitive mouth, or stomodaeum. The globular processes at first stand 

 distinctly apart from each other, and each globular process is separated from 

 the corresponding maxillary process by a groove which leads to the nasal 

 pit of that side. 



The lateral nasaL process is separated from the globular process by the 

 nasal pit, and externa' to the lateral nasal process is the optic or ocular de- 



