BONES OF THE SKULL 51 



physial plate develops between it and the body of the axis. The anterior arch of the atlas 

 represents a cartilaginous hypochordal bar, which is present in the early stages of development 

 of the vertebrae, but disappears in all but the atlas in the ossification of the body. 



The arches and spinalis processes are easily recognised throughout the various parts of the 

 column in which complete vertebrae are present. 



The articular processes or zygapophyses are of no morphological value, and do not require 

 consideration here. 



The transverse processes offer more difficulty. They occur in the simplest form in the tho- 

 racic series. Here they articulate with the tubercles of the ribs, whence the term tubercular 

 processes or diapophyses has been given them (the place of articulation of the head of the rib 

 with the vertebra is the capitular process or parapophysis), and the transverse process and the 

 neck of the rib enclose an arterial foramen named the costo-transverse foramen. In the cer- 

 vical region the costal element (pleurapophysis) and the transverse process are fused together, 

 and the conjoint process thus formed is pierced by the costo-transverse foramen. The com- 

 pound nature of the process is indicated by the fact that the anterior or costal processes in the 

 lower cervical vertebrae arise from additional centres and occasionally retain their independence 

 as cervical ribs, and in Sauropsida (birds and reptiles) these processes are represented by free 

 ribs. In the lumbar region, the compound nature of the transverse process is further marked. 

 The true transverse process is greatly suppressed, and its extremity is indicated by the accessory 

 tubercle. Anterior to this in the adult vertebrae a group of holes represents the costo-transverse 

 foramen, and the portion in front of this is the costal element. Occasionally it persists as an 

 independent ossicle, the lumbar rib. 



In the sacral series the costal elements are coalesced in the first three vertebrae to form the 

 greater portion of the lateral portion for articulation with the ilium, the costo-transverse fora- 

 mina being completely obscured. In rare instances the first sacral vertebra will articulate with 

 the ilium on one side, but remain free on the other, and under such conditions the free process 

 exactly resembles the elongated transverse process of a lumbar vertebra. The first three sacral 

 vertebrae which develop costal processes for articulation with the ilium are termed true sacral 

 vertebrae, while the fourth and fifth are termed pseudo-sacral. A glance at fig. 64 will show the 

 homology of the various parts of a vertebra from the cervical, thoracic, lumbar, and sacral 

 regions. 



B. BONES OF THE SKULL 



The skull is the expanded upper portion of the axial skeleton and is supported 

 on the summit of the vertebral column. It consists of the cranium, a strong bonj^ 

 case enclosing the brain and made up of eight bones — viz., occipital, two parietal, 

 frontal, two temporal, sphenoid, ethmoid; and the bones of the face, surrounding 

 the mouth and nose, and forming with the cranium the orbital cavity for the 

 reception of the eye. The bones of the face are fourteen in number — ^viz., two 

 maxillce, two zygomatic {malar), two nasal, two lacrimal, two palate, two inferior 

 conchce (turhinates) , the mandible, and the vomer. All the bones enumerated 

 above, with the exception of the mandible, are united by suture and are therefore 

 immovable. The proportion between the facial and cranial parts of the skull 

 varies at different periods of life, being in the adult about one (facial) to two 

 (cranial), and in the new-born infant about one to eight. A group of movable 

 bones, comprising the hyoid, suspended from the basilar surface of the cranium, 

 and three small bones, the incus, malleus, and stapes, situated in the middle ear or 

 tympanic cavity, is also included in the enumeration of the bones of the skull. 



According to the BNA nomenclature, the term cranium is used in a wider sense as synony- 

 mous with skull, and is subdivided into cranium cerebrale (cranium in the narrower sense) and 

 cranium viscerate (facial skeleton). In the BNA, seven bones above listed with the facial, — 

 two inferior conchae, two lacrimal, two nasal and the vomer — are classed with the cranium 

 cerebrale. 



THE OCCIPITAL 



The occipital bone [os occipitale] (fig. 65) is situated at the posterior and 

 inferior part of the cranium. In general form it is flattened and trapezoid in 

 shape, curved upon itself so that one surface is convex and directed backward 

 and somewhat downward, while the other is concave and looks in the opposite 

 direction. It is pierced in its lower and front part by a large aperture, the foramen 

 magnum, by which the vertebral canal communicates with the cavity of the 

 cranium. 



The occipital bone is divisible into four parts, basilar, squamous, and two 

 condylar, so arranged around the foramen magnum that the basilar part hes in 

 front, the condylar parts on either side, and the squamous part above and behind. 



( 



