Page:Encyclopædia Britannica, Ninth Edition, v. 1.djvu/881

Rh DEVELOPMENT.] ANATOMY 831 vertebra, for they lie in front of, and not behind, the vertebral notches through which the spinal nerves are transmitted. The development of the odontoid process of the axis shows it to be the lody of the atlas displaced from its proper bone and fused with the body of the axis. The development and homology of the skull is a much more difficult problem to solve than that of the spine. The chorda dorsalis extends along the floor of the skull as far forward as the posterior wall of the pituitary fossa. Cartilage is formed around it, without, however, the previous production of proto-vertebrce, and this cartilage is prolonged forward on each side of the fossa, forming two bars, the trabeculce cranii ; these bars then unite, and form the mes-ethmoid cartilage ; at the same time the cartilage grows out wards for some distance in the membranous wall of the skull, but it does not mount upwards so as to close it in superiorly, so that the cartilage is limited to the floor of the skull ; moreover, the cartilage is not segmented. The roof, side walls, and anterior wall of the cranium retain for a time their primordial membranous struc ture. This membrane is prolonged downwards into the face proper, where it forms a pair of maxillary lobes or processes, which pass forwards beneath the eyes to form the side parts of the face, and a mid- or frontal-nasal process, into which the cartilaginous mes- ethmoid extends. Immediately below each maxillary lobe four arches, called branchial or visceral, arise in the ventral aspect of the head, and in each of the three first of these arches a rod of cartilage is formed. The arches on opposite sides unite with each other in the ventral mesial line, but those on the same side are separated from each other by intermediate branchial clefts; these clefts all close up in course of time, except the upper part of the first, which remains as the external meatus of the ear, the tympanum, and the Eustaehian tube ; whilst the interval between the first visceral arch and the maxillary lobes forms the cavity of the mouth. The con version of the primordial cartilaginous and membranous cranium into the bones of the head takes place by the formation in it of numerous centres of ossification. The basi-, ex-, and so much of the supra-occipital as lies below the superior curved line, are formed from distinct centres in the cartilaginous floor of the skull ; whilst the part of the supra-occipital above the curved line arises from independent .centres in the membranous cranium, the whole ulti mately fusing together to form the occipital bone. The basi- or post-sphenoid, the pre- with the orbito-sphenoids, the ali-spheuoid with the external pterygoid and the internal pterygoid, also arise in the cartilaginous floor, and they, together with the sphenoidal spongy bones which are formed in the membranous cranium, fuse into the sphenoid bone. The palate is apparently formed by ossifica tion of cartilage continuous with the bar in which the internal pterygoid arises. The central plate and each lateral mass of the ethmoid also arise in the cartilage by distinct centres. The inferior turbiual has also a distinct origin in cartilage. The petro-mastoid part of the temporal arises in cartilage from at least three centres, peri-, pro-, and opisth-otic, and soon blends with the squamous and tympanic elements which arise in the membranous cranium ; subsequently the styloid process, which is ossified in the rod of cartilage in the second visceral arch, joins the temporal. The lower end of this same rod forms the lesser cornu of the hyoid ; the upper end forms two small bones, the stapes and incus, situated within the cavity of the tympanum. The cartilage of the third visceral arch forms the great cornu and body of the hyoid bone. The name of Meckel s cartilage is applied to the rod found in the first visceral arch ; its upper end is ossified into the malleus, a small bone situated in the tympanic cavity ; whilst in the membrane sur rounding the rest of the cartilage the lower jaw-bone is formed. The parietal and frontal bones arise altogether in the membranous vault ; and the nasal, lachrymal, malar, and superior rnaxillte arise in connection with the bones which form the face ; the vomer is developed in the membrane investing the mes-ethmcid cartilage. The human superior maxilla represents not only the superior maxilla of other vertebrates, but the pre-maxillary bone also ; but the two bones become fused together at so very early a period that it is difficult to recognise their original independence. In the deformity of hare-lip and cleft palate, they are not unfrequently separated by a distinct fissure. Since the time when Oken and Goethe propounded the theory that the skull was built up of several vertebrae, the vertebral struc ture of the skull has led to much discussion amongst anatomists. Every one admits that the skull is in series with the spine, that the cranial cavity is continuous with the spinal canal, and that the cranial vault is formed in the wall of the embryonic cerebro-spinal canal. The skull also, like the spine, is transversely segmented, but whether we regard these segments as vertebra? or not will depend upon the conception we entertain of the meaning of the term vertebra. If with Owen wo define a vertebra to be &quot;one of those segments of the endo-skeleton which constitute the axis of the body and the protective canals of the nervous and vascular trunks,&quot; then we may support the vertebral nature of the cranial segments on the following grounds : 1st, The presence of a series of bones extending forwards from the foramen magnum along the basis cranii, in series with the bodies of the spinal vertebrse, e.g., the basi-occipital, basi-sphenoid, pre-sphenoid, rues-ethmoid (Fig. 7) ; 2d, The presence of a series of neural arches which enclose and complete the wall of the cranial cavity, and lie in series with the neural arches of the spinal vertebra, e.g., the ex- and supra-occi- pitals, which form the neural arches of the basi-occipital segment ; the ali-sphenoids and parietals, which form the neural arches of the basi-sphenoid segment ; the orbito-sphenoids and frontal, which form the neural arches of the pre-sphenoid segment ; 3d, The presence of a series of visceral arches of which the mandibular and hyoidean enclose the alimentary and vascular canals, just as the ribs enclose them in the thorax ; and 4th, The presence of foramina between the cranial segments like the inter-vertebral foramina between the spinal vertebrce for the transmission of nerves, e.g., the sphenoidal fissure and the jugular foramen. But if we are to regard a vertebra as a segment of the axial skeleton, which in course of its formation passes through a definite series of developmental changes, then the cranial segments cannot be regarded as vertebra in the same sense as the spinal segments ; for, 1st, The chorda dorsalis is not co-equal in length with the basis cranii, as with the bodies of the spinal vertebrse, so that if the basi-occipital and basi-sphenoid segments, the bodies of which are developed around it, were to be regarded as cranial vertebras, the pre-sphenoidal and ethmoido-nasal would not be morphologically the same, as they are formed in front of the anterior end of the chorda. 2d, Proto-vertebrse are formed in the spine, but not in the basis cranii. 3d, The spine is transversely segmented in its cartilaginous stage of development, but the skull is not. 4th, The transverse segmentation of the skull only appears when the bones are formed, but the individuality of the segments becomes again concealed by the fusion of the pre- and basi-sphenoids and the basi-occipital into a continuous bar of bone, a condition which is not found in the spine except in the sacro-coccygeal region. 5th, The neural arches in the spine are, like the bodies, ossified in cartilage, but in the cranium they are for the most part ossified in membrane. These differences in the mode of development of the spine and basis cranii may be summarised as below: Spine. 1st Stage, 2d Stage, 3d Stage, 4th Stage, Unsegmeiited Proto-vertebra. Segmented Segmented chorda. cartilage. bones. Basis Cranii. 1st Stage, 2d Stage, 3d Stage, 4th Stage, Unsegmeiited Unsegmeiited Segmented Unsegmented chorda in part. cartilage. bones. bones. It is evident, therefore, that, although both skull and spine are developed in the walls of the cerebro-spinal groove, yet, to quote the words of Huxley, &quot;though they are identical in general plan of construction, the two begin to diverge as soon as the one puts on the special character of a skull and the other that of a vertebral column ; the skull is no more a modified vertebral column than the vertebral column is a modified skull. &quot; The limbs, at their first appearance, sprout like little buds or lappets from the sides of the trunk ; cartilage forms within them, which assumes the shape of the future bones, and as the limbs grow outwards, manifestations of joints appear, and the subdivision of each limb into its several segments takes place. The clavicle, which ossifies before any of the other bones, begins to form, how ever, in fibrous membrane ; and at a much later period the ends of the bone, which are formed in cartilage, unite with the intermediate shaft. The scapula ossifies from one centre for its expanded plate and spine, two small centres each for the acromion and vertebral border, and one for the coracoid. In many vertebrates, more espe cially birds and reptiles, the coracoid is a distinct bone from the scapula, but they articulate with each other to form the glenoitf fossa. Each of the three rod-like bones of which the innomirmt&amp;lt; bone is composed, ossifies from one centre for the shaft of the bone, and one for each extremity ; in the ilium these terminal centres an situated at the crest and acetabulum ; in the ischium, at the tuber and acetabulum ; and in the pubis, at the symphysis and acetabu lum. Each of the long bones of the shafts of the limbs ossifies from a single centre for the shaft, and one or more centres for each articular extremity. Each carpal and tarsal bone ossifies from a single centre, except the os calcis, which possesses an independent centre for its posterior surface. The metacarpal and metatarsal bones and the phalanges ossify each from two centres, one for the shaft and one for one of the extremities. In the metacarpal bones of the fingers and the four outer metatarsals, the distal end is that which ossifies independently ; in the metacarpal of the thumb, in the meta- tarsal of the great toe, and in all the phalanges, the proximal end is that which ossifies independently. As the method of ossification o the first metacarpal and first metatarsal corresponds with that of tha phalanges, some anatomists hold that these bones are really the first phalanges of their respective digits, and that the bone which is absent in these digits, when compared with the other digits, is not a phalanx, but a meta-carpal or tarsal bone. When tho extremity