THE SKULL AS A WHOLE 



141 



this outer wall are two marked projections of bone (Figs. 81 and 104). One is 

 known as the turbinated bone and the other as the middle turbinated process. The 

 superior turbinated process appears as a less distinct bony projection. This sur- 

 face presents three irregular longitudinal passages, termed the superior, middle, 

 and inferior meatuses of the nose (Figs. 104 and 105). The superior meatus 

 (meatus nasi superior), the smallest of the three, is situated at the upper and back 

 part of each nasal fossa, occupying the posterior third of the outer wall. It is 

 situated between the superior and middle turbipated processes, and has opening 

 into it two foramina, the sphenopalatine foramen at the back of its outer wall, and 

 the posterior ethmoidal cells at the front part of the outer wall. The sphenoidal 

 sinus opens into a recess, the spheno-ethmoidal recess (recessus sphenoethmoidalis}, 

 which is situated above and behind the superior jkurbinated process. The middle 

 meatus (meatus nasi medius) is situated external to the middle turbinated process, 

 and above the turbinated bone, and extends from the anterior end of the turbinated 

 to the sphenopalatine foramen of the outer wall of the nasal fossa. Anteriorly 

 jt terminates in a depression, the atrium of the nasal meatus. The middle meatus 

 presents in front the orifice of the infundibulum (infundibulum ethmoidale), by 

 which the middle meatus communicates with the anterior ethmoidal cells, and 

 through these with the frontal sinuses. The posterior ethmoidal cells also open 

 into this meatus, while at the centre of the outer wall is the orifice of the maxillary 

 antrum which varies somewhat as to its exact position in different skulls. The 

 inferior meatus, the largest of the three, is the space between the turbinated bone 

 and the floor of the nasal fossa. It extends along the entire length of the outer 

 wall of the nose, is broader in front than behind, and presents anteriorly the lower 

 orifice of the canal for the nasal duct. 



Pituitary 

 fossa 



Mesoderm of base 

 of skull 



ParacJwrdal 

 cartilage 



Development of the Skull. Up to a certain stage the development of the skull corresponds 

 with that of the vertebral column; but it is modified later in association with the expansion of 

 the brain vesicles, the formation of the organs of smell, sight, and hearing, and the development 

 of the mouth and pharynx. 



The notochord extends as far forward in the 

 base of the future skull as the anterior end of the 

 mid-brain, and becomes partly surrounded by 

 mesoderm (Fig. 107). The posterior part of this 

 uesodermal investment corresponds with the 

 future basiocciput, and shows a subdivision into 

 four segments, which are separated by the roots 

 of the hypoglossal nerve. The mesoderm then 

 extends over the brain vesicles, and thus the entire 

 brain is enclosed by a mesodermal investment, 

 which is termed the membranous primordial cra- 

 nium. From the inner layer of this the bones of 

 the skull and the membranes of the brain are de- 

 veloped ; from the outer layer the muscles, blood- 

 vessels, true skin, and subcutaneous tissues of the 

 scalp. In the shark and dog-fish this membran- 

 ous cranium undergoes complete chondrification, 

 and forms the cartilaginous skull, or chondro- 

 cranium, of these animals. In mammals, on the 

 other hand, the process of chondrification is 

 limited to the base of the skull the roof and 

 sides being covered in by membrane. Thus, 

 the bones of the base of the skull are preceded 



by cartilage, those of the roof and sides by membrane. The posterior part of the base of the 

 skull is developed around the notochord, and exhibits a segmented condition analogous to 

 that of the vertebral column, while the anterior part arises in front of the notochord and shows 

 no regular segmentation. The base of the skull may therefore be divided into (a) a chorda! or 

 vertebral, and (6) a prechordal or prevertebral portion. 



In the lower vertebrates two pairs of cartilages are developed, viz., a pair of parachordal 

 cartilages, one on either side of the notochord; and a pair of prechordal cartilages, the trabeculae 



Anterior arch of atlas 

 Notochord 

 Body of axis 



Third cervical 

 vertebra 



FIG. 107. Sagittal section of cephalic end of 

 notochord. (Furness). 



