110 OSTEOLOGY. 
the end of the second month of feetal life. The notch for the hypoglossal canal appears 
about the third month. From this centre is formed the posterior three-fourths of the 
occipital condyle. The exoccipital is usually completely fused with the squama by the 
third year or earlier. 
As already noted, the squama consists of two parts—the one above the occipital crest, 
the other below it; the former develops in membrane, the latter in cartilage. In a 
three-months foetus this difference is very characteristic. The cartilaginous part (supra- 
occipital) begins to ossify from two centres about the sixth or seventh week, which 
rapidly join to form an elongated strip placed transversely in the region of the occipital 
protuberance. The centres for the age part (interparietal) appear later. According to 
Magei (Arch. Ital. Biol. tome 26, fas. 2, p. 301), they are four in number, of which two 
placed on either side of the middle line appear about the second month. The other pair, 
placed laterally, are seen about the third month ; fusion between these takes place early, 
but their disposition and arrangement explain the anomalies to which this part of the 
bone is subject. The mesial pair may persist as separate ossicles, or fuse to form the 
pre-interparietals, whilst the lateral pair may remain independent of the supraoccipital as 
a single or double interparietal bone. Union between the supraoccipital and the inter- 
parietal elements occurs about the third or fourth month ; but evidence of their separation — 
is frequently met with even in the adult by the persistence of a transverse suture running 
inwards from each external angle of the squama, or, as above mentioned, there may be an 
os Inc. The supraoccipital forms a small part of the middle of the hinder border of the 
foramen magnum, though here a small independent centre, known as the ossicle of 
Kerkring, is occasionally met with. Other independent centres are sometimes seen 
between the supraoccipital and the exoccipitals. 
At birth the occipital consists of four parts—the interparietal and supraoccipital 
combined, the basioccipital, and the exoccipitals—one on either side. 
THE TEMPORAL BONES. 
The temporal bone (0s temporale) hes about the centre of the lower half of either 
side of the skull, and enters largely into the formation of the cranial base. It is 
placed between the occipital behind, the parietal above, the sphenoid in front, and 
the occipital and sphenoid internally and below. At birth it consists of three 
parts—an upper and outer part, the squamous or squamo-zygomatic portion ; an inner 
and posterior portion, the petro-mastoid, which contains the organ of hearing, 
together with that specially associated with equilibration ; and an under or tympanic 
part, from which the floor and anterior wall of the external auditory meatus is 
formed. 
The squamous part (pars squamosa) consists of a thin shell-like plate of bone 
placed vertically, having an inner (cerebral) and an outer (temporal) surface and a 
semicircular upper border. Inferiorly, behind, and internally it is fused in early 
life with the petro-mastoid portion by means of the squamoso-mastoid and the 
petro-squamosal sutures, traces of which are often met with in the adult bone; 
whilst below and in front it is separated from the tympanic and petrous parts by 
the Glaserian fissure. Its external surface, smooth and shghtly convex, enters into 
the formation of the floor of the temporal fossa, and affords attachment to the 
temporal muscle. Near its hinder part it is crossed by one or more ascending 
erooves for the branches of the middle temporal artery. In front and below there 
springs from it the zygomatic process (processus zy gomaticus). This arises by a 
broad attachment, the surfaces of which are inferior and superior ; curving outwards 
and forwards, it then becomes twisted and narrow so that its sides are turned 
inwards and outwards and its edges directed upwards and downwards. Anteriorly 
it ends in an oblique serrated extremity which articulates with the zygomatic pro- 
cess of the malar bone. Posteriorly the edges of this process separate and are 
termed its roots. The upper edge, which becomes the posterior root, sweeps back 
over the external auditory meatus, and is confluent with a ridge, the supra-mastoid 
crest, which curves backwards and slightly upwards, and serves to define the limit 
of the temporal fossa posteriorly. The inferior edge turns inwards and constitutes 
the anterior root; the under surface of this forms a transversely-placed rounded 
ridge, the articular eminence (tuberculum articulare), behind which there is a deep 
