176 OSTEOLOGY. 
the length of the cranio-facial axis has been found by Flower to be a character of much value. 
The dental length is taken by measuring the distance beween the anterior surface of the first pre- 
molar and the posterior surface of the third molar of the upper jaw. 
To obtain the dental index the following formula is used :— 
ental length x 10 A 
Des - enst St ° _ Dental index. 
Basinasal length 
Following the convenient method of division adopted with other indices, the dental indices 
may be divided into three series s, called respectively— 
Microdont, index below 42: including the so-called Caucasian or white races. 
Mesodont, index between 42 and 44: “including the Mongolian or yellow races. 
Megadont, index above 44: comprising the black rac eS, meluding ‘the Australians. 
Many complicated instruments have been devised to take the various measurements required, 
but for all practical purposes the calipers designed by Flower or the compas glissiére of Broca are 
sufficient. 
As an aid to calculating the indices, the tables published in the Osteological Catalogue of the 
Royal College of Surgeons ” of Engla nd, Part L, Man., or the index calculator invented by Dr. 
Waterston will be found of much service in sav ing time. 
DEVELOPMENT OF THE CHONDRO-CRANIUM AND MORPHOLOGY 
OF THE SKULL. 
As has been already stated (p. 23), the chorda dorsalis extends forwards to a point 
immediately beneath the anterior end of the mid-brain. In front of this the head takes a 
bend so that the large fore-brain overlaps the anterior extremity of the notochord. At 
this stage of development the cerebral vesicles are enclosed in a membranous covering 
derived from the mesenchyme surrounding the notochord ; this differentiated mesodermal 
layer is called the primordial membranous cranium. From this the meninges which 
invest the brain are derived. In lower vertebrates this membranous capsule becomes 
converted into a thick-walled cartilaginous envelope, the primordial cartilaginous cranium. 
In mammals, however, only the basal part of this capsule becomes chondrified, the roof and 
part of the sides remaining membranous. In considering the chondrification of the skull 
in mammals, it must be noted that part only of the base is traversed by the notochord. It 
is, therefore, conveniently divided into two parts—one posterior, surrounding the notochord, 
and hence called chordal, and one in front, into which the notochord does not extend, and 
hence termed prechordal. ‘These correspond respectively to the vertebral and evertebral 
regions of Gegenbauer. In the chordal region a pair of elongated cartilages, called the 
parachordal cartilages, appear one on either side of the notochord ; these soon envelop the 
chorda, and expand so as to form the basilar or occipital plate, which ossifies later to form 
the basilar process of the occipital bone, and the dorsum sella of the sphenoid. In the 
prechordal region two curved strips of cartilage, the trabecule cranii of Rathke, arise and 
pass for wards from the anterior extremity of the notochord, one on either side of the 
cranio-pharyngeal canal. In front these trabeculee spread out and ultimately fuse to form 
the ethmoidal plate, which constitutes the fore part of the chondro-cranium. Posteriorly 
the trabeculee unite with the basilar plate and thus surround the cranio-pharyngeal canal, 
the lumen of which is subsequently closed to form the floor of the pituitary fossa, in which 
rests the hypophysis cerebri. Owing to the presence of the nasal capsules, the fore part 
of the ethmoidal plate becomes differentiated into an ethmo-vomerine region, from which 
the nasal septum and its associated cartilages are derived, whilst the remainder of the 
ethmoidal plate by expansion and subsequent ossification develops to form the pre- 
sphenoid, the orbito-sphenoids, and the alisphenoids, which latter assist in completing 
the orbital cavity for the lodgment of the eyeball. The membranous ear capsules which 
lie lateral to the parachordal cartilages become chondrified and form the cartilaginous 
ear capsules. These soon unite with the lateral aspects of the basilar plate, but are 
separated in front from the cartilaginous alisphenoid of the ethmoidal plate by a 
membranous interval, which is subsequently occupied by the squamosal, a bone of dermic 
origin. This disappearance of the cartilage under the squamosal was regarded by Parker 
as the diagnostic mark of the mammalian chondro-cranium. 
From the ventral surface of this cartil: aginous platform—formed, as described, by ihe 
union of the trabeculie, parachordal cartilages, and cartilaginous ear capsules—is suspended 
the cartilaginous framework of the visceral arches, which play so important a part in the 
development of the face, an account of which is elsewhere given (p. 36). 
A consideration of the facts of comparative anatomy and embryology appears to justify 
