GENERAL MORPHOLOGY OF THE BONES OF THE HEAD. 81 



the tympanic plate, while its Vidian branch occupies a canal surrounded by the basisphenoid, 

 alisphenoid and pterygoid elements of the sphenoid bone. The glosso-pharygeal (ninth), 

 pneumo-gastric (tenth) and spinal accessory (eleventh) nerves pass between the periotic and 

 exoccipital ; and the hypoglossal (twelfth) between the exoccipital and basioccipital. The 

 internal carotid arteiy, it may be added, enters the skull by the foramen lacerum, a space 

 between the basisphenoid, alisphenoid, and periotic, having previously traversed the carotid 

 canal formed by the downgrowth of the opisthotic ; and the jugular vein issues between 

 the periotic and exoccipital. 



A general review of the construction of the skull shows that it may be regarded as consist- 

 ing mainly of three sets of parts, viz. 1st, basal or central parts, comprising the basi- 

 occipital, basisphenoid, presphenoid, and mesethmoid, which form a series prolonged forwards 

 in the line of the vertebral axis, and constitute a crania-facial axis ; 2nd, superior arches, 

 three in number, enclosing the brain, and consisting of more or less expanded bones, viz., the 

 exoccipitals and supraoccipital together with the interparietal, the alisphenoids, squamosals and 

 parietals, the orbitosphenoids and f rentals ; and 3rd, inferior arches, surrounding the visceral 

 cavity as represented by the nose, mouth, and pharynx ; these include the pterygoids, palatals 

 and maxillae in a first arch, the mandible and the malleus of the internal ear in a second, the 

 cerato-, epi-, stylo- and tympanohyals in a third, while the thyrohyals are the rudiments of a 

 fourth, and the basihyal is interposed between the last two. To these succeed in the lowest 

 vertebrates the series of branchial arches, one, or perhaps two, of which may be in part 

 represented by the thyroid cartilage of the larynx. Together with the foregoing there are 

 associated other elements, viz. 1st, the periotic enclosing the organ of hearing, and the 

 ethmo- and maxilloturbinals covering the organ of smell, which have been classed sepa- 

 rately as special sense-capsules ; and 2nd, the small bones of the face, malar, nasal, 

 and lachrymal, supplementary to the maxillary arch, and the vomer, extending the 

 cranio-f acial axis. 



There is thus a certain resemblance in the arrangement of the chief parts of the skull to 

 that of the trunk-skeleton, and this resemblance has led to the conception of what is called 

 the vertebrate theory of the skull, according to which the skull consists essentially of a 

 series of vertebras, the dorsal or neural arches of which have undergone great expansion, so 

 as to predominate over the less developed ventral or visceral arches. Certain circumstances 

 in the growth of the skull, especially the formation of a part at least of the cranio-facial axis 

 around a prolongation of the notochord, appear at first sight to give support to this view ; 

 but the more complete knowledge of the mode of development of the skull which has been 

 obtained of late years tends to show that there is no such homodynamous correspondence 

 between the several bones of the skull and the vertebral segments of the trunk-skeleton. In 

 explanation of this statement the following points of difference between the two may be 

 specially referred to, viz. 1, a large part of the cranio-facial axis is prechordal, being formed 

 beyond the cephalic extremity of the notochord, and therefore does not correspond to vertebral 

 bodies ; 2, the cartilage in which the bones of the cranio-facial axis and its lateral expansions 

 are developed is not at any period segmented, as is the cartilage preceding the vertebras ; 3, 

 the segmentation which is to be recognized in the bones of the skull does not agree with the 

 segmentation which has been observed in the head of lower vertebrates at an early period of 

 embryonic life ; 4, the bones forming the dorsal and ventral arches of the head are in large 

 part developed in membrane, and are probably dermal in origin, whereas the arches of the 

 vertebras are entirely of cartilaginous origin ; and 5, the cartilaginous visceral arches of the 

 head probably do not correspond to the costal arches of the vertebras, being formed in close 

 connection with the wall of the alimentary canal, while the ribs are developed in the body- 

 wall or somatopleure. 



On the whole, it would appear, therefore, that the skeleton of the head and the skeleton 

 of the trunk agree in being formed in continuous tissue lying between and surrounding the 

 cerebro-spinal axis and the alimentary canal, and that the correspondence observed in their 

 general construction is due to this community of origin and relations. But the several bones 

 are developed independently, and in a different manner in the head and trunk, giving rise in 

 the one case to the skull, in the other to the vertebral column ; and although the head as a 

 whole is undoubtedly derived in large part from a portion of the body that was primitively 

 segmented, there is no evidence to show that the appearance of the skull has been preceded 

 by a stage in which osseous or even cartilaginous vertebras are developed in connection with 

 these segments. 1 (Huxley, "Lectures on Comp. Anat.," 1864; Gegenbaur, "Elements of 

 Comparative Anatomy," and Morph. Jahrb., xiii ; O. Hertwig, " Lehrb. der Entwicklungs- 

 geschichte.") 



1 The foetal conditions referred to above are fully explained in the section on Embryology in 

 Vol. I. 



voi. ir. a 



