MOKPHOLOGY OF THE LIMBS. 293 



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 basi- 

 sphenoid and basi-occipital are still united by cartilage. The cartilage also which blocks the 

 foramen lacerum may be regarded as a remnant of the chondro-cranium. 



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. 



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 bodies of which corresponded to the basi-cranial axis, whilst 

 the vertebral 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 vertebrae, the number of 

 which he determined by noting the metameric arrangement of the cerebral 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 cerebral nerves, 

 are excluded, since from the nature of their development they are to be regarded as meta- 

 morphosed 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 cerebral 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 greatly- 

 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 vertebrae 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 cerebral nerves, and in the 

 fundament of the visceral skeleton (visceral arches). According to Froriep, the mammalian 

 occipital corresponds to the fusion of four vertebrae, and there is some reason for supposing that 

 in some classes of vertebrates the occipital region of the primordial cranium is increased by 

 fusion with the higher cervical segments. 



The form of skull characteristic of man is dependent on the large proportionate development 

 of the cranial part, which contains the brain, and the reduction in size of the visceral part 

 (face), which protects the organs of special sense. This leads to a decrease in the mass and 

 projection of the jaws, as well as a reduction in the size of the teeth. Associated with the 

 smaller mandible there is a feebler musculature, with a reduced area of attachment to the sides 

 of the skull. In this way the disappearance of the muscular crests and fossae, so characteristic 

 of lower forms, is accounted for. At the same time the fact that the skull is poised on the 

 summit of a vertical column, leads to important modifications in its structure. The disposition 

 of parts is such that the occipito-vertebral articulation is so placed that the fore and hind parts 

 of the head nearly balance each other, thus obviating the necessity for a powerful muscular and 

 ligamentous mechanism to hold the head erect. 



Another noticeable feature in connexion with man's skeleton is the prolonged period during 

 which growth may occur before maturity is reached ; this is associated with a more complete 

 consolidation of the skull, since bones, which in lower forms remain throughout life distinct, are 

 in man fused with each other, as exemplified in the case of the presphenoid and postsphenoid, 

 the occipital and the interparietal, to mention one or two instances among many. It is 

 noteworthy, however, that during ontogeny the morphological significance of these bones is 

 clearly demonstrated by their independent ossification. 



The points of exit of the various cerebral nerves remain remarkably constant, and in their 

 primitive condition serve to suggest the segmental arrangement of the cartilaginous chondro- 

 cranium already referred to. Owing to the very great modifications which the mammalian skull 

 has undergone in the process of its evolution, it may be pointed out that the passage of the 

 nerves through the dura mater a derivative, the readers may be reminded, of the primordial 

 membranous cranium (see ante) alone represents the primitive disposition of the nerves. Their 

 subsequent escape through the bony base is a later and secondary development. In some cases 

 the two, membranous or primary and the osseous or secondary foramina, correspond. In other 

 instances the exit of the nerves through the dura mater does not coincide with the passage 

 , through the bone. 



Of interest in this connexion it may be pointed out that the foramina and canals which 

 traverse the skull are either situated in the line of suture between adjacent bones or in the line 

 of fusion of the constituent parts of which the bone pierced is made up. For example, the 

 superior orbital fissure is situated between the orbito and alisphenoids ; the hypoglossal canal 

 between the basi and exoccipitals ; the jugular between the petrous, basi, and exoccipital ; 

 the optic between the orbito-sphenoid and the presphenoid ; the pterygoid between the alisphenoid, 

 medial pterygoid lamina, and the lingula. 



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