10 THOMAS DWIGHT ON THE 



The long-itudiiial arrangement is evident in the seal (fig. 27), l)ut transverse sections 

 of the lumbar vertebrae show the plates diverging from the compact bone on the inner 

 side of the pedicle with remarkable clearness. There is also a set rimning horizontally 

 inward from the lower surface of the transverse process which arises from the body of 

 the vertebra. These vertebrae present another peculiarity: to ^vit, the great thickness 

 ol" the epiphyses at the ends of the bodies which consist of cancellae arranged longitu- 

 dinally. Tliis description applies to the Groenlandica as Avell as to the P. foetida. In 

 one or two dorsal vertebrae of the Otaria, which I had an opportunity to examine, the 

 longitudinal system was clear and the transverse indistinct. In all these there were the 

 thick plates and large spaces which are characteristic of the seal tribe. 



The vertebrae of cetaceans shoAV the characteristic sti'ucture of many thin plates vei'y 

 close together. The arrangement is clearly longitudinal with tubular spaces. In the 

 dorsal region the structure is very dense and heavy, the middle of the body in a small 

 wdiale being almost solid bone. The structure is lighter in the lumbar region and in the 

 caudal lighter still. The systems of horizontal plates from the transverse processes can 

 be traced but a short distance in the body. The lumbar vertebra of the whale (fig. 28) 

 has one peculiarity that I have seen in neither carnivora nor ungulata, namely, a solid 

 crust of bone surrounding the body. The iipper one is continuous wdth the compact 

 tissue of the pedicles, the lower Avith that of the transverse processes. In the dorsal I'egion 

 this solid shell is found only at the upper side of the vertebra bounding the canal. I 

 have not found this in either the dorsal or lumbar region of the porpoise. 



I have had the oijportunity of cutting but one bone of the Slrenia, one of the anterior 

 lumbo-caudal vertebrae of the manatee (fig. 29). The structure is very whale-like. It 

 has the same plate siTrrounding the body as in the same region in glohicephalas ; 

 the same decussation of the plates from the transverse process on each side ; a substantia 

 intermedia and at the lowest part a structure of coarser plates and larger spaces. 

 Above this on either side of the middle the structure is denser than in the Imnbar ver- 

 tebra of the whale. 



Sunnning up our observations on the bodies of the mammalian vertebrae we find that 

 the plan is in the main a longitudinal one. In man, the structure is the lightest of all; 

 in whales and the manatee the heaviest. There can hardly be a doubt that the pressure 

 to w^hich the bodies of the vertebrae are constantly subjected is in an antero-posterior 

 direction being due to the comjjression they exercise one on another. In quadrupeds 

 the dorso-lnmbar region may be compared to a bridge,^ one end of which is supported 

 on piers (to wit, the femurs), the other being suspended (from the scapulae) sometimes 

 only by muscles, sometimes having the additional support of a claAdcle. The thorax is 

 suspended by the serratus, the neck by the levator angiili scapulae and therefore the tho- 

 racic spine is sujiported through the riljs. The pieces of the spine are connected by 

 strong longitudinal ligaments; many and powerful muscles, by their contraction, press 

 the vertebrae together, hence the longitudinal arrangement of the plates, for it is neces- 

 sary to the safety of the arch that it should be compressed. In man, the arrangement 

 is different. The spine is placed on end and really requires less strength than in the 

 quadruped. 



' Compare Liicae; Kobbe uiiJ Otter. AI)haiKlliiiigeu lies Seutkcub. natiirf. Ges., Bil. viii ami ix. 



