CARTILAGINOUS SKULL OF A 21 MM. HUMAN EMBRYO. 315 



attached several muscles, namely, the occipitomastoid, the atlantomastoid, the 

 digastric, the stapedius, and the longissimus capitis (figs. 14 and 15). 



The anterior part of the cartilaginous skull; the sphenoid and the ethmoid, 

 bears very little resemblance to anything ordinarily seen in the lateral view of the 

 adult skull, partly because most of the bone ossified in cartilage becomes covered 

 or shut out from the lateral view by membrane bone. If we can imagine all of 

 this membrane bone stripped away, the resemblance between the two would be 

 clearly seen. 



The lateral surface of ths bady of the sphenoid, with the shallow sella turcica 

 odging the hypophysis, is to be seen in front of the apex of the cochlear part of the 

 otic capsule. The prominent dorsum sellse projects towards the great mid-brain 

 fissure, and from it (but not shown) the thin medial membranous part of the ten- 

 torium projects into this fissure. The temporal wing lies for the most part below 

 the level of the basisphenoid and is attached to the under surface of the alar 

 process, while the orbital wing lies, for the most part, above the level of the basis- 

 phenoid and is curved around the optic nerve. The two parts of the orbital 

 wing are clearly indicated in figures 7 and 14. In the description of the middle 

 cranial fossa and the embryonic tentorium cerebelli it was noted that the lateral 

 prismatic mass was so placed that the apical edge extended upward from the 

 dorsum sellse in connection with the thin medial membranous part of the tentorium 

 lying in the great mid-brain fissure. The lower end of the embryonic tentorium, 

 as previously noted, occupies the space between the otic capsule and the orbital 

 wing of the sphenoid, the semilunar ganglion filling up most of the gap (fig. 8). 

 It lies outside of the cartilaginous skull but within the membranous skull. The 

 geniculate ganglion and adjacent sections of the facial nerve are not incased in 

 cartilage, and they also he in this middle cranial fossa or gap (figs. 12 and 13). 



In the lateral aspect of the chondrocranium most of the mesethmoid is shut 

 out from view by the nasal capsule. Between the orbital wing and the nasal capsule 

 the anterior end of the basisphenoid and a bit of the mesethmoid appear. If only 

 the cartilaginous skull were considered, this region of the basisphenoid might be 

 regarded as interorbital and the term interorbital septum could be applied. But 

 this so-called interorbital septum exists only in the cartilaginous skull; in the 

 more complete skull, with all the blastema and precartilage, the medial wall of 

 the orbit is complete enough to shut it out from the orbit. The term interorbital 

 septum probably has no significance whatever in human embryology. The 

 prominent crista galli projects between the cerebral hemispheres. The lateral 

 surface of the nasal capsule forms a large proportion of the inner wall of the orbit 

 and probably ossifies into the lamina papyracea of the ethmoid. 



THE OCCIPITAL CARTILAGE. 



The occipital cartilage consists of an elongated, flattened body (or basioc- 

 cipital), the lateral parts (or exoccipitals) , and the two nuchal plates (or squamse). 



The body forms the greater part of the so-called basal plate, a term much used 

 in descriptions of the cartilaginous skull. It continues without line of demarcation 



