On Pneumatic Foramina in Fossil Bones. 69 



^ suggestion, or only words, for no law will produce anatomical 

 ejBFects without corresponding physiological circumstances to sculpture 

 the bones. 



" If the influence of pneumatic pressure produces a well-known 

 osteological result in excavation of a bone in a bird, what is there 

 in the vertebrae of a Dinosaur to suggest that similar effects have 

 been produced by dissimilar causes ? And it would be interesting 

 to find in an extinct order of animals evidence that an agency 

 unconnected with the lungs produced results which differ from 

 those in birds only in being the effect of larger areas of pressure 

 acting laterally upon the sides of the vertebrae. But the evidence 

 that thei'e was any essential difference in the origin of these 

 structures in Dinosaurs and birds is not forthcoming ; and if it ever 

 existed is lost with the soft parts of the animal. 



" Nevertheless, cavities are formed in certain bones in animals of 

 varied organization, which are not connected with the lungs in the 

 manner of air-cells of birds, but they ai'e chiefly in the skull. They 

 are slightly developed in existing reptiles, but are most conspicuous 

 in warm-blooded animals. The skulls of elephants exhibit a 

 maximum development of pneumatic cavities which have no 

 connexion with the lungs, and the texture of these bones closely 

 approximates to that of cellular vertebrae in some Cetiosaurian 

 Dinosaurs, such as Ornithopsis and its American representatives. 

 The resemblance between the mammal skull and the reptile 

 vertebra is one of analogy. There are no facts to support the 

 inference that the cause which expanded the cranial bones of the 

 elephant and other niammals is identical with that which absorbed 

 and excavated the bony tissue, but did not augment the size of the 

 cervical and dorsal vertebrae of Dinosaurs. There is no basis for 

 comparison between the conditions in mammals and these extinct 

 reptiles, for no mammal shows a pneumatic vertebral column which 

 can be compared with these Dinosaurs ; and when a mammalian 

 vertebra is hollow it is not comparable, since there is no pneumatic 

 foramen. 



" On the other hand, Dinosaurs are not conspicuous for pneumatic 

 cavities in bones of the skull, and there are therefore no facts to 

 suggest the idea that they might by analogy develop a pneumatic 

 vertebral column which was not connected with the lungs, even if 

 •the cranial and vertebral pneumatic structures had been comparable. 



" The influence of the lungs as a whole in modifying the vertebral 

 column of a reptile is manifest in the dorsal vertebrae of Testudinata. 

 In tortoises, under conditions of terrestrial life, the lungs have 

 ■expanded and given the carapace a remarkable elevation. At the 

 same time the neural arches have become raised, and the lungs 

 have pressed evenly against the sides of the centra of the vertebra 

 till they have become narrowed into thin plates by the tissue being 

 absorbed laterally. But the lung never penetrates into the 

 substance of the vertebra or excavates holes in the bones in existing 

 reptiles comparable to those seen in skeletons of Dinosaurs, 

 Ornithosaurs, and birds. 



