450 MANUAL OF ZOOLOGY. 



opinion as to the exact composition of the skull. There is, 

 however, a very general concurrence of opinion that the skull 

 is composed of a series of separate segments, and this is a 

 point which it is important to remember. By Owen, and by 

 many other competent authorities, these cranial segments are 

 looked upon as being nothing more than so many vertebrce, 

 the neural canals of which are greatly expanded to enclose 

 the brain, whilst the haemal arches are very greatly modified to 

 serve different purposes. This view is not accepted by high 



Fig. 241. Skeleton of an Armadillo, showing the regions of the vertebral column, c 

 Cervical region ; d Dorsal region ; / Lumbar region ; s Sacral region ; t Caudal 

 region or tail. 



authorities ; but the general fact that the skull is composed of 

 separate segments is universally admitted. The only portion 

 of the bony framework of the head which it is absolutely 

 essential to understand, is the lower jaw or "mandible." The 

 lower jaw is sometimes wanting, but when present, it consists 

 in all Vertebrata of two halves or " rami," which are united to 

 one another in front, and articulate separately with the skull 

 behind. In many cases, each half, or " ramus," of the lower 

 jaw consists of several pieces united to one another by sutures; 

 but in the Mammalia each ramus consists of no more than a 

 single piece. The two rami are very variously connected with 

 one another, being sometimes only joined by ligaments and 

 muscles, sometimes united by cartilage or by bony suture, and 

 sometimes fused or anchylosed with one another, so as to leave 

 no evidence of their true composition. The mode by which 

 each ramus of the lower jaw articulates with the skull also 

 varies. In the Mammalia the lower jaw articulates with a 

 cavity formed on what is known to human anatomists as the 

 temporal bone; but in Birds and Reptiles. the lower jaw articu- 



