MORPHOLOGY OF THE SKULL. Lt 
the assumption that the mammalian skull is of two-fold’ origin— that, im fact, (it. 18 
composed of two envelopes, an outer and an inner, primarily distinct, but which in the 
process of evolution have become intimately fused together. The inner, called the 
primordial skull, is that which has just been described, and consists of the chondro- 
cranium and the branchial skeleton. The outer, which is of dermic origin, includes the 
bones of the cranial vault and face which are developed in membrane. This secondary 
skull, which first appears in higher fishes as ossified dermal plates overlying the primary 
skull, acquires a great importance in the mammalia, as owing to the expansion of the 
brain and the progressive reduction of the chondro-cranium, these dermal bones become 
engrafted on and incorporated with the primordial skull, and act as covering bones to the 
cavities of the cranium and face; for it may be well to point out that these dermal or 
membrane bones are not necessarily external in position, as over the cranial vault, but 
also develop in the tissues underlying the mucous membrane of the cavities of the face. 
Advantage is taken of this difference in the mode of development of the bones of the 
skull to classify them according to their origin into cartilage or primordial bones, and 
membrane or secondary bones. These differences in the growth of the bone must not be 
too much insisted on in determining the homologies of the bones of the skull, as it is now 
generally recognised that all bone is of membranous origin, and that whilst in some 
cases cartilage may become calcified, it never undergoes conversion into true bone, but is 
replaced by ossific deposit derived from a membranous source. In the subsequent growth 
of the skull parts of the cartilaginous cranium persist as the septal and alar cartilages of 
the nose, whilst for a considerable period the basisphenoid and basioccipital are still 
united by cartilage. 
Till two years after birth there are membranous intervals between the edges and 
angles of the bones of the cranial vault. These are termed the fontanelles. Normally 
they are six in number, and correspond in the adult to the position of the bregma and 
lambda in the middle line and the pterion and asterion on either side. The anterior or 
bregmatic fontanelle is diamond-shaped, and corresponds to the converging angles of the 
parietals and two halves of the frontal bone. The posterior fontanelle is triangular in 
form, and lies between the two parietals and the summit of the occipital squama. The 
antero-lateral fontanelle lies between the contiguous margins of the frontal, parietal, 
squamous temporal, and great wing of the sphenoid, whilst the postero-lateral fontanelle 
is situated between the adjacent borders of the parietal, occipital, and mastoid portion of 
the temporal. 
Whilst in many instances the primordial and secondary bones remain distinct in the 
fully-developed condition, they sometimes fuse to form complex bones, such as the temporal 
and sphenoid (see pp. 116 and 123). 
Various theories have been advanced to account for the mode of formation of the 
skull. The earliest of these was called the vertebrate theory, which assumed that the 
cranium was built up of a series of modified vertebrae, the centra of which corresponded to 
the basicranial axis, whilst the neural arches were represented by the covering bones of 
the cranium. In view of the more recent researches regarding the composite origin of the 
skull above referred to, this theory was necessarily abandoned. It gave way to the 
suggestion of Gegenbauer that the primordial cranium has arisen by the fusion of several 
segments equivalent to vertebra, the number of which he determined by noting the 
metameric arrangement of the cranial nerves, of which he concluded there were nine pairs, 
arranged much like spinal nerves, both as to their origin and distribution. The olfactory 
and optic nerves, though frequently referred to as cranial nerves, are excluded, since from 
the nature of their development they are to be regarded as metamorphosed parts of the 
brain itself. Gegenbauer therefore assumed that that portion of the cranial base which is 
traversed by the nine pairs of segmentally arranged cranial nerves must be formed by the 
fusion of nine vertebral segments ; and as the region where the nerves escape corresponds 
to the part of the chondro-cranium traversed by the notochord, he calls it the vertebral 
portion of the cranial base, in contradistinction to the trabecular or non-vertebral part 
which lies in front. This latter he regards as a new formation adapted to receive the 
ereatly-developed brain and afford protection to the organs of sight and smell. 
As has been pointed out by Hertwig, there is an essential difference between the 
development of the axial cartilaginous skeleton of the trunk and head. The former becomes 
segmented into distinct vertebree alternating with intervertebral ligaments ; whilst the 
latter, in order to attain the rigidity necessary in this part of the skeleton, is never so 
divided. It follows from this that the original segmentation of the head is only expressed 
in three ways, viz. in the appearance of several primitive segments (myotomes), in the 
arrangement of the cranial nerves, and in the fundament of the visceral skeleton (viscera! 
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