IN THE HUMAN FRAME. 61 



quadrupeds the number of vertebrae is from thirty to forty, 

 in the serpent it is nearly one hundred and fifty ; where- 

 as in men and quadrupeds the surfaces of the bones are 



crum is as the stejy on which the base of the pillar, like the heel of the 

 mast, is socketed or morticed. The spine is tied to the lateral parts of 

 the pelvis by powerful ligaments, which may be compared to the 

 shrouds. They secure the lower part of the spine against the shock 

 of lateral motion or rolling ; but, instead of the stays to limit the play 

 of the spine forwards and backwards in pitching, or to adjust the rake 

 of the mast, there is a very beautiful contrivance in the lower part of 

 the column. 



The spine forms here a semicircle which has this effect ; that wheth- 

 er by the exertion of the lower extremities, the spine is to be carried 

 forward upon the pelvis, or whether the body stops suddenly in run- 

 ning, the jar which would necessarily take place at the lower part of 

 the spine, if it stood upright like a mast, is distributed over several ot 

 the bones of the spine ; and, therefore, the chance of injury at any 

 particular part is diminished. 



For example, the sacrum, or centre bone of the pelvis, being car- 

 ried forward, as when one is about to run, the force is communicated 

 to the lowest bone of the spine. But, then, the surfaces of these 

 bones stand with a very slight degree of obliquity to the line of mo- 

 tion ; the shock communicated from the lower to the second bone of 

 the vetebrae is still in a direction very nearly perpendicular to its sur- 

 face of contact. The same takes place in the communication of force 

 from the second to the third, and from the third to the fourth ; so that 

 before the shock of the horizontal motion acts upon the perpendicular 

 spine, it is distributed over four bones of that column, instead of the 

 whole force being concentrated upon the joining of any two. 



If the column stood upright, it would be jarred at the lowest point 

 of contact with its base. But by forming a semicircle, the motion 

 would produce a jar on the very lowest part of the column, and which 

 is distributed over a considerable portion of the column; and in point 

 of fact, this part of the spine never gives way. Indeed, we should be 

 inclined to offer this model to the consideration of nautical men, as 

 fruitful in hints for improving naval architecture. 



Every one who has seen a ship pitching in a heavy sea, must have 

 asked himself why the masts are not upright, or rather, why the fore- 

 mast stands upright, whilst the main and mizen masts stand oblique to 

 the deck, or, as the phrase is, rake aft or towards the stern of the ship. 



The main and mizen masts incline backwarks, because the strain is 

 greatest in the forward pitch of the vessel ; for the mast having re- 

 ceived an impulse forwards, it is suddenly checked as the head of the 

 ship rises ; but the mast being set with an inclination backwards, the 

 motion falls more in a perpendicular line from the head to the heel. 

 This advantage is lost in the upright position of the foremast, but it is 

 sacrificed to a superior advantage gained in working the ship ; the 

 sails upon this mast act more powerfully in swaying the vessel round, 

 and the perpendicular position causes the ship to tack or stay better ; 

 but the perpendicular position, as we have seen, causes the strain 

 in pitching to come at right angles to the mast, and is, therefore, more 

 apt to spring. 



These considerations give an interest to the fact, that the human 

 spine, from its utmost convexity near its base, inclines backwards, " 

 Bell's Treatise on Animal Mechanics. 



