MOKPHOLOGY OF THE SKULL. 



291 



which comprises the basilar portion of the occipital bone. From this there pass extensions 

 which form the lateral parts of the occipital bone, and serve to unite the occipital plate, as this 

 part of the cartilaginous base' is 'sometimes called, to the cartilaginous auditory capsules on 

 either side. These latter are formed by the chondrification of the cochlear and canalicular parts 

 of the labyrinth, which do not develop at the same rate, so that the part around the semi- 

 circular canals is completed much sooner than the cochlear portion ; in consequence, at the end 

 of the second month, the facial nerve and the genicular ganglion lie in a groove, to be subse- 

 quently converted into a canal, on the vestibular part of the capsule. 



The dorsal part of the ring consists of a thin cartilaginous plate, the tectum posterius, from which 

 is developed the only part (i.e. the inferior part of the occipital squama) of the cranial vault 

 preformed in cartilage. In the membranous tissue from which this plate is developed chondrifica- 

 tion at first begins, on either side, by an extension from the posterior aspect of the pars lateralis 

 of the occipital; growing rapidly forwards this ultimately unites with the posterior and dorsal 

 borders of the cartilaginous auditory capsule, from which it is for some time separated by a 

 narrow membranous interval. At a later stage the cartilages of either side unite, dorsal to the 

 foramen magnum, to form the tectum posterius or the tectum synoticum (Keibel and Mall). 



To the axial part of this portion of the chondrified base the chorda dorsalis has the following 



Basi-sphenoid centres 



Pre-sphenoid centre 



^ Frontal 



Orbito-sphenoid 

 - Ali-splienoid 



' Ali- sphenoid 



Squamous part 

 of temporal 



). 283. OSSIFICATION ON BASE AND LATERAL WALLS OF SKULL OF FOUR AND A HALF MONTHS' 

 FCETUS (Schultze's method). Cartilage, blue ; cartilage-bone, black ; membrane-bone, red. 



relations : in front of the foramen magnum it runs for a short distance in a groove on the dorsal 

 surface of the occipital plate, then pierces the cartilage so as to lie ventral to it in the retro- 

 pharyngeal tissue, again enters the chondro - cranium by passing dorsalwards in the suture 

 between the occipital plate and sphenoidal cartilage and ends dorsal to the latter cartilage. 



The prechordal portion of the cartilaginous basis cranii in man displays the following features : 

 at the third month it is irregularly diamond-shaped in outline, its posterior angle is wedged in 

 between the two auditory capsules and is related to the anterior part of the axial portion of the 

 occipital plate. The anterior angle forms the ventral end of the nasal capsule, whilst the lateral 

 angles extend over the orbital cavities and correspond to the tips of the alse orbitales of the 

 sphenoid. 



Within this area chondrification takes place as follows (Bardeen). In the region of the 

 posterior angle, above referred to, a cartilaginous nodule appears anterior and ventral to the end 

 of the chorda dorsalis ; from this arises the cartilaginous body of the sphenoid, the further 

 development of which is associated with its union with the anterior end of the median portion 

 of the occipital plate and the formation there, by the appearance of an independent transverse 

 strip of cartilage, of the dorsum sellse of the sphenoid ; whilst from its anterior and superior 

 surface on either side there extend forwards strips of cartilage which surround the hypophyseal 

 pocket, and unite in front of it to form the anterior part of the body of the sphenoid, thereby 

 enclosing the hypophyseal canal, which, at first wide, is gradually closed by the chondrification 

 of its walls. It may, however, remain open. 



j.y a 



