126 HIGH SCHOOL ZOOLOGY. 



regions present in all the higher Vertebrates ; of these, the cervi- 

 cal contaijis thirteen vertebrae, and it alone retains any freedom 

 of movement between the individual bones, for the vertebrae of 

 the trunk and the greater part of the tail are for the most part 

 so united by bone that no such movement is possible. The last 

 few vertebrae are, however, movable, and the terminal one, the so- 

 called 2)y<jod.yh (which is really formed by the fusion of several 

 centra) ser\'es to support the tail-feathers. The chief charac- 

 tei'istic, then, of the bird's vertebral column is that the iliac bones, 

 instead of being united with a few sacral vertebrae as in the 

 reptiles, have acquired a union with the vertebrae in front and 

 behind these, so that the whole region is stiffened into one mass. 

 It is obvious that such an arrangement is well calculated to 

 transmit the whole weight of the body to the hind limbs. 



A couspicuous difference between the higher and the lower Vetebrates 

 is in the nature of tlie union between the skull and the vertebral colunui. 

 In the tish the basi-oceipital closely resembles and forms a part of a 

 series with the vertebral centra ; in the Amphibian, on the other hand, 

 the ex-occipitals form two surfaces of contact (occipital condyles) with 

 the vertebral column, but in the higher Vertebrates greater freedom 

 of moveiuent is conferred upon the head by the specialisation of the first 

 two vertebrai for this purpose. The anterior or atlas possesses surfaces 

 for receiving the one (Sauropsida) or two (^Lammalia) occi^jital condyles, 

 and it itself rotates on a pivot (tlie odontoid process — which is really 

 part of the centrum of the Atlas — ) attached to the centrum of the 

 second vertebra or axis. 



As for the skull, the chief thing to notice is the early fusion of 

 its component pai'ts in such a way as to make it iniDossible to 

 distinguish the limits between the cranial bones, although the 

 sutures between those of the face ai'e easily seen. The })reuiax- 

 ilhe are of large size, and chiefly support the upper beak, (which 

 is movable lo a certain extent on the skull behind), while the 

 maxillae are insignificant and are united to the outer surface of 

 the movable quadrate by a slender bar formed of the jugal and 

 quadra to-jugal. The quadi-ate moves partly ou the skull and 



