760 



MAMMA 



Fig. 382. 



(2180.) The anterior, or frontal vertebra (c) has for its body the an- 

 terior sphenoid (alee minores) ; its arch being completed by the cavity of 

 the os frontis, which encloses anteriorly 

 the cribriform plate of the ethmoid bone. 



(2181.) From this analysis of the 

 composition of the cranium, it is appa- 

 rent that the temporal bones, although 

 in Man they assist so materially in com- 

 pleting the cranial cavity, are only in- 

 tercalated between the real vertebral 

 elements ; as indeed might almost have 

 been anticipated, seeing how differently 

 the pieces belonging to this bone are ar- 

 ranged in different classes of Vertebrata. 



(2182.) Such is the general organi- 

 zation of the Mammiferous skeleton. 

 Let us now proceed to consider the os- 

 teology of the different orders into which 

 the Mammalia have been distributed, 

 and observe in what respects they in- 

 dividually differ from each other. 



(2183.) The transition from Birds to 

 Quadrupeds, remotely separated as they 

 might appear to be, is effected by gentle 

 gradations of structure ; and the MONO- 

 TKEMATA, notwithstanding their quadru- 

 pedal form and hairy covering, are so 

 nearly allied to the feathered Ovipara 

 in many points of their organization, 

 that they evidently form a connecting 

 link between these two great classes of 

 animals. 



(2184.) It is true that they have 

 mammary glands, and must therefore be supposed to give suck to their 

 offspring ; but it is not even yet satisfactorily determined whether they 

 lay eggs, or produce living young. The structure of their generative 

 apparatus would seem, in fact, to be rather allied to the Oviparous than 

 the Mammiferous type ; and, as in Birds, the rectum, the urinary pas- 

 sages, and the sexual organs, all discharge themselves into a common 

 cloacal chamber; so that there is still but a single vent a circumstance 

 from which the name of the order is derived. 



(2185.) Even their skeleton, in many points, presents a very close 

 affinity to that of a Bird, as will be evident on examining the osseous 

 system of the Orniihorhynclius paradoxus (fig. 383). 



(2186.) The mouth of this quadruped, indeed, resembles that of a 



Cranial vertebrae. 



