THE INTERNAL SKELETON, 179 
the Apteryx, the anterior margin of the sternum is strongly 
concave. It is slightly so in the Rhea. 
The median process extending posteriorly is called the middle 
xiphoid process, and if there is a single backwardly projecting 
process on either side of it that is called the lateral wiphoid 
process. If, instead of one, there are two such processes on 
either side of the “middle xiphoid” process, then the more 
external process of each such lateral pair is termed the external 
lateral «iphoid process, and the other the internal lateral aiphord 
process. 
Other names have been imposed on parts of the sternum. 
Thus the median part which supports the keel has been called 
the lophosteon; the anterior lateral piece which receives the 
ribs has been named the plewrosteon ; and the part on either side 
which forms the single or double lateral xiphoid process has 
been distinguished by the term metosteon, 
The hinder end of the keel and the part bearing it may 
remain more or less cartilaginous, as we see in the Fowl. 
: 
THE SKULL AND ITs APPENDAGES. 
~ The Cranial Skeleton is made up of three parts, separable by 
mere removal of the soft tissues. These are (1) the skull with 
the upper jaw, (2) the lower jaw, and (3) the bones of the 
tongue or the os hyotdes. . 
The Skull consists mainly of a rounded bony box, the cranium 
proper, containing the brain, with a hole behind through which 
the brain and the spinal marrow become continuous. The skull 
also affords protection to the organs of sight and hearing. The 
latter organs are enclosed within the substance of the lower, 
hinder part of the skull-wall, near a conspicuous external 
opening which leads towards them. The organs of sight are 
sheltered and partially enclosed behind and above, by the bony 
framework of the antero-lateral and anterior parts of the brain- 
case, the roof of which—or frontal part of the skull—projects 
forwards and also outwards over the bony orbits, or large lateral 
concavities for the reception of the eyeballs. From the front of 
the base of the brain-case a long tapering and pointed process, 
the rostrum, extends forwards. A thin vertical plate of bone 
ascends from this rostrum to the roof of the orbits, between 
which it may, or may not, form a complete, vertical, antero- 
posteriorly extending partition or septum. 
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