858 



PACHYDERMATA. 



these osseous masses are generally small, and 

 sometimes without material density, the indi- 

 vidual granules not having firmly united. In 

 fact they resemble in every point adventitious 

 bone. 



If in a young animal the fracture be not 

 kept tolerably quiet, the motion between the 

 fractured bone will prevent the formation of 

 cartilage, which seems necessary to the deve- 

 lopement of bone, and here, therefore, osseous 

 masses will alone present themselves. This 

 fact is very interesting in a surgical point of 

 view, and might be treated more at length ; but 

 having given a detailed account of the deve- 

 lopement of bone and of the distinction to be 

 made between true bone and adventitious 

 osseous tissue, I shall conclude the article. 



(J. Tomes.) 



OVARY. See SUPPLEMENT. 

 OVUM. See SUPPLEMENT. 



PACHYDERMATA. An extensive group 

 of herbivorous quadrupeds, constituting a dis- 

 tinct order of the class Mammalia, generally 

 remarkable for their ponderous bulk and un- 

 wieldy appearance, and seemingly forming the 

 transition between the gigantic Cetacea, which 

 from their size are only adapted to an aquatic 

 existence, and the vegetable-eating Mammals 

 of strictly terrestrial habits. Even the localities 

 where they are met with would seem to indi- 

 cate that they constitute such a connecting 

 link, seeing that their most typical forms are 

 peculiarly adapted to be occupants of the river 

 and the marsh, from the Hippopotamus, that 



might almost be considered an aquatic animal, 

 to the Tapirs and the Hog, which still love to 

 wallow in mud although they approximate in 

 their habits to the ruminating quadrupeds. At 

 the present day the order Pachydermata con- 

 tains but few genera, and these for the most 

 part embrace a very limited number of species. 

 But in former periods of the history of our 

 globe they must have existed under much 

 greater variety of form, seeing that the tertiary 

 deposits yield to the geologist, in abundance, 

 the remains of very numerous genera now 

 totally extinct, to the list of which modern 

 researches are adding day by day ; it is indeed 

 more than probable that many of the existing 

 races will speedily perish, for the hand of man 

 is against them, and the bullet and the spear 

 are doing their work of extermination rapidly, 

 so that the Tapir and the Elephant, like the 

 Paloeotherium and the Mastodon, may soon be 

 classified with extinct existences. 



The order of Mammalia under consideration 

 is usually divided into PROBOSCIDIANS, in- 

 cluding such Pachydermata as are provided 

 with a proboscis and tusks, of which the Ele- 

 phant is the only existing example, and into 

 ORDINARY PACHYDERMATA, which are unpro- 

 vided with a proboscis, and characterized by 

 possessing four, three, or two large digits on 

 their feet, which are cased in horny hoofs the 

 last group being distinguishable from the Rumi- 

 nantia by the simple construction of their 

 stomachs, although closely approaching them 

 in many points of their economy. The above 

 division, however useful to the zoologist, is 

 nevertheless by no means based on nature, the 

 proboscis of the Elephant being only a maximum 



Fig. 464. 



Skeleton of Elephant ( Elephas Indicus). 



