376 MANUAL OF ZOOLOGY. 



number of vertebrae which constitute the " sacral region," and 

 are usually amalgamated or " anchylosed " together to form a 

 single bone, the " sacrum " (s). 5. The spinal column is 

 completed by a variable number of vertebrae which constitute 

 the " caudal " region, or tail (/). 



As regards the skull of the Vertebrata, it has been thought 

 advisable not to enter into any general details here, partly 

 because the subject is one which can only be properly dis- 

 cussed in a work specially devoted to Human or Comparative 

 Anatomy, and partly because there is still much diversity of 

 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 number 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 vertebra, 

 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 Hux- 

 ley, but the general fact that the skull is composed of separate 

 segments appears to be 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 

 articulates with the skull, not directly, but by the intervention 

 of a special bone, known as the " quadrate bone " or os quad- 

 ratum. 



As regards the limbs of Vertebrates, whilst many differences 

 exist, which will be afterwards noticed, there is a general 

 agreement in the parts of which they are composed. As a 

 rule, each pair of limbs is joined to the trunk by means of a 



