66 CHARACTERS OF VERTEBRATE ANIMALS 



cases such a gland consists of an aggregate of blindly-ending tubes 

 which are ingrowths of the epidermis. As a general rule, these 

 tubes open externally upon a teat, but in the two lowest known 

 Mammals this is not the case, there being a simple depression of 

 the skin, within which milk-pores are seen. 



Besides the two important characters just dealt with, the 

 Mammalia are distinguished by others of scarcely less value, which 

 may be conveniently dealt with under the headings employed when 

 describing the anatomy of the human subject. 



Endoskeleton (see pp. 25-32). -Two very noteworthy features 

 enable one to distinguish the skull of a Mammal from that of other 

 Vertebrates. One is the presence of two occipital condyles at the 

 back, and the other is the fact that the lower jaw is made up of 

 only two pieces united together in front. Birds and Reptiles 

 possess only one such condyle, and this, in cases where a well- 

 developed neck is present, allows the head a very free motion, 

 as may be observed in a bird, though in this particular case the 

 large range of movement is also in part due to the existence of 

 very perfect joints in the neck itself. Amphibia, e.g. the frog, 

 also have two occipital condyles, but their skulls differ from those 

 of Mammalia in so many other ways that no one is likely to be 

 led into error thereby. As to the nature of the lower jaw, it is 

 made up of a considerable number of pieces in Vertebrates lower 

 than Mammals. 



The backbone of a Man is typically Mammalian in its main 

 features, except that the tail region is much more reduced than is 

 usually the case. We note that the individual joints are flat- 

 ended, and that in the young state the body or centrum of each 

 is made up of a central piece, to which a bony disc is united at 

 each end. This character applies to all but the very lowest 

 members of the group. A further typical feature, to which but 

 very few exceptions are known, is found in the fact that the 

 vertebrae of the neck are seven in number, even in a giraffe. 



The skeleton of the human limbs is typical in many ways, the 

 chief modifications of a special nature being seen in the structure 

 of the lower limbs, especially as regards the peculiar arches of the 

 pelvis. Such peculiarities are adaptations to the requirements of 

 the erect attitude. But the various bones recognizable in Man 

 are recognizable also in most Mammals, and it may in particular 

 be remarked that the possession of five fingers and five toes is the 



