Natural History 497 



apart from the main part of the bone ; the surfaces of the vertebrae 

 are usually flat or gently rounded ; with four exceptions there are 

 seven neck vertebra; whether it be in the long straight neck of the 

 giraffe or the compressed inconspicuous neck of the whale; the 

 lower jaw is one bone on each side and works on a bone of the skull 

 called the squamosal; the skull moves by two knobs (or condyles) 

 on the first vertebra?, whereas birds and reptiles have only one con- 

 dyle; the drum of the ear is connected with the internal organ 

 of hearing by a beautiful chain of three small bones the hammer, 

 the anvil, and the stirrup by which the vibrations are conveyed 

 inwards; there is a complete bony palate separating the mouth- 

 cavity from the nasal passage above; almost without exception 

 there are two sets of teeth in sockets ; except in the oviparous mam- 

 mals the bone of the shoulder-girdle called the coracoid, which 

 is very strongly developed in flying birds and in reptiles, is repre- 

 sented merely by a small process of the shoulder-blade or scapula. 

 The cerebral hemispheres of the fore-brain are much more 

 developed than in other vertebrates, and their surface is very gen- 

 erally covered with convolutions ( see figure facing p. 157, Vol. I) . 

 The heart is four-chambered ; the temperature of the blood remains 

 in most cases practically constant ; the red blood-cells are circular 

 discs (except in camels, where they are elliptical in outline as in 

 other Vertebrates), and the nucleus of the mammalian red blood 

 corpuscle disappears as the corpuscle develops; the lungs lie 

 freely in the chest cavity (they are fixed in birds) , and inspiration 

 is the active process (the opposite in birds) ; the vocal cords are at 

 the top of the windpipe (at the foot in birds) ; the egg-cells are 

 very small except in the egg-laying forms, and, with the same 

 exception, the young are born viviparously, i.e., as living well- 

 formed young ones, which are for a while nourished on milk. 

 This enumeration of salient characters is indispensable if we are 

 to understand how this class of Mammals stands apart from the 

 other great classes of backboned animals, namely, Birds, Reptiles, 

 Amphibians, and Fishes. Aristotle knew that a whale is not a 



