THE INTEENAL SKELETON. 1 79 



the Apteryx, the anterior margin of the sternum is strongly 

 concave. It is slightly so in the Ehea. 



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 xiphoid 

 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 xiphoid process, and the other the internal lateral xiphoid 

 process. 



Other names have been imposed on parts of the sternum. 

 Thus the median part which supports the keel has been called 

 the hphosteon ; the anterior lateral piece which receives the 

 ribs has been named the pleurosteon ; 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 Setjll 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 hyoides. 



Ths 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-waU, near a cons,picuous 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. Prom 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. 



N 2 



