SIGNIFICANCE OF BONE STRUCTUEE. H 



In whales the conditions are not quite tlie same; nevertheless the spine is not straight 

 but bent in the thoracic region. The shape of the bodies of the vertebrae is not such 

 that this could be done without the restraining force of ligaments and the vast power 

 of the muscles must pull the vertebrae together; hence, here also the longitudinal 

 arrangement. 



In the seal the plan is less modified. It has been stated that the pressm-e is trans- 

 mitted to the thoracic portion of the spine through the ribs and very probably this oc- 

 curs through the tubercle that ai'ticulates with the transverse process much more than 

 through the head of the rib. Hence the pedicles connecting the transverse processes 

 with the body must be strong. But this is not the only reason; the pedicles have also 

 to transmit the strain, probably very considerable, occasioned by the pressure on the 

 articular processes in various movements. Thus the firm walls which the pedicles us- 

 ually present are what might be expected, and it is natural enough that plates should 

 pass off from these points of support through the vertebral bodies both lengthwise and 

 crosswise. Still, as a rule, the work done b}' these systems does not seem important. 

 Before ajiproaching some other questions, it may be interesting to take a cursory view 

 of the vertebrae of other classes. 



In the osseous fishes (fig. 30) the body is doubly concave. The end plates are sup- 

 ported by others running antero-posteriorly which appear as radiating lines in a trans- 

 verse section. These are connected by occasional transverse plates which in some places 

 are so developed as to siiggest concentric rings. 



In the alligatoi- (fig. 31), the plan bears a certain resemblance to that of the fish. 

 There is a general radiating arrangement with mostly large interspaces. Near the up- 

 per part of the body, i. e., just below the canal, the structure is thicker. The cancellous 

 tissue is veiy dense in the rounded end. Longitudinal sections show plates running 

 from before backward and also near the side, strong plates running downward and 

 backward from the anterior articular processes, and others downward and forward from 

 the posterior ones. There is a beautiful strong system of plates with large interspaces 

 in the arches and roots of the transverse processes that is very characteristic. 



In the great Galapagos tortoise a cervical vertebra shows a plan of rather thick plates 

 and large interspaces. The osseous tissue is much less dense than in the alligator. 

 The vertebra cut being the only cervical one in the series, it is not worth while to discuss 

 the arrangement in detail as it cannot well be compared with the others. Sutfice it to 

 say that the plates are disposed chiefly longitudinally, and that the general appearance 

 is not in the least like that of the alligatoi-. 



The vertebi-a of the snake is curious, presenting more solid substance than any verte- 

 bra I am acquainted with. Figs. 33 and 34 represent two frontal sections made respec- 

 tively near the back and front of the body of a vertebra of a pithon. Fig. 34 is just 

 behind the hollowed anterior end. The circular portion of spongy tissue is just back of 

 the cup and is inclosed by an envelope of compact bone. The latter is found also at 

 the lowest part of the body and in the articular processes which receive the posterior 

 ones of the vertebra in front. The other section shows almost exclusively solid tissue 

 both in the body and in the posterior articular processes. Fig. 32 shows a horizontal 

 section through the body. The spongy tissue on the convex head is well shown; that in 



