650 MAMMALIA [CH. 



apparently only the Mammal has an apparatus for analysing a sound- 

 disturbance into its component parts. The outer ear or ectodermal 

 involution leading to the drum is in Mammalia guarded by an 

 external lobe termed the pinna of the ear. 



The neck region of Mammals (with rare exceptions) always 

 consists of seven vertebrae, and thus whereas a long-necked Bird 

 like a Swan has numerous short vertebrae in this region, in a 

 long-necked Mammal like the Giraffe the same region consists of 

 seven immensely long vertebrae. The sternum of Mammals also is 

 peculiar, consisting of distinct pieces orsternebrae. The first of 

 these is called the presternum, and bears a crest for the attach- 

 ment of the pectoral muscles ; the last ends in a spade-like xiphoid 

 cartilage, and is called the xiphisternurn. The intervening 

 segments constitute the sternebrae of the mesosternum. The 

 lower ends of a pair of ribs are attached opposite the junction of 

 two sternebrae (Fig. 321). The division of the sternum into seg- 

 ments is supposed to be due to the strains exercised on it by the 

 sternal ribs owing to their movements in aid of respiration. 



In Mammalia as in other Amniota the centrum is formed from 

 the ventral intercalaries, and the head of the rib is articulated 

 between two vertebrae ; the articulation is not shifted on to the 

 vertebra as in Crocodilia. The vertebrae have occasionally in the 

 neck region cup and ball articulations lit > those of Amphibia, 

 Reptilia and Aves, but elsewhere the thick intervertebral cartilage 

 allows of sufficient bending, and the centra have flat ends which ossify 

 late and for some time form separable discs of bone called epip hy ses. 

 These do not represent any new elements in the centrum, but only 

 a method of ossification found in the limb-bones of Amphibia, 

 Reptilia and Aves as well as in Mammalia. This method is as 

 follows: the ensheathing membrane or periosteum first forms a 

 tube of bone round the centre of the cartilage, the ends of which re- 

 main soft and capable of further growth; these terminal "epiphysial" 

 cartilages are only replaced by bone when growth is diminishing, 

 and are united with the main ossification when growth has ceased. 



In the fore-limbs of Mammals the chief point to be noticed is 

 the reduction in size of the pectoral girdle to which the fore- 

 limbs are attached. The lower part of this, the coracoid, which 

 in Bir.ds and Reptiles is a large, strong bone meeting the sternum, 

 is here, with the exception of a few primitive forms, a small hook 

 with no connection at all with the sternum. Hence the pectoral 

 girdle is much more movable than is elsewhere the case, and takes 



