CO ON MECHANICAL ARRANGEMENT 



siderably varied ; but with a strict reference to the conveni- 

 eiicy of the animal. (PI. IX. fig 3, 4, 5.) For, whereas in 



being balanced. We, therefore, see the great advantage resulting 

 from the human spine being in the form of an italic/. It is prepared 

 to yield in the direction of its curves ; the pressure is of necessity more 

 upon one side of the column than on the other ; and its elasticity is 

 immediately in operation without a jerk. It yields, recoils, and so forms 

 the most perfect spring ; admirably <;alculated to carry the head with- 

 out jar, or injury of any kind. 



The most unhappy illustration of all this is the condition of old age. 

 The tables of the skull are then consolidated, and the spine is rigid : 

 if an old man should fall with his head upon the carpet, the blow, 

 which would be of no consequence to the elastic frame of a child, may 

 to hiin prove fatal ; and the rigidity of the spine makes every step 

 which he takes, vibrate to the interior of the head, and jar on the brain. 



We have hinted at a comparison betwixt the attachment of the spine 

 to the pelvis and the insertion of the mast of a ship into the hull. The 

 mast goes directly through the decks without touching them, and the 

 heel of the mast goes into the step, which is formed of large ^solid 

 pieces of oak timber laid across the keelson. The keelson is an inner 

 keel resting upon the floor-timbers of the ship and directly over the 

 proper keel. These are contrivances for enlarging the base on which 

 the mast rests as a column ; for as, in proportion to the height and 

 weight of a column, its base must be enlarged, or it would sink into 

 the earth ; so, if the mast were to bear upon a point, it would break 

 through the bottom of the ship. 



The mast is supported upright by the shrouds and stays. The 

 shrouds secure it against the lateral or rolling motion, and the stays 

 and backstays against the pitching of the ship. These form what is 

 termed the standing rigging. 



The mast does not bear upon the deck or on the beams of the ship ; 

 indeed there is a space covered with canvass betwixt the deck and the 

 mast. 



We often hear of a new ship going to sea to stretch her rigging ; that 

 is, to permit the shrouds and stays to be stretched by the motion of the 

 ship, after which they are again braced tight ; for if she were overtak- 

 en by a storm before this operation, and when the stays and shrouds 

 were relaxed, the mast would lean against *^^he upper deck, by which 

 it would be sprung or carried away. Indeed, the greater proportion 

 of masts that are lost are lost in this manner. There are no boats 

 which keep the sea in such storms as those which navigate the gulf 

 of Finland. Their masts are not attached at all to the hull of the 

 ship, but simply rest upon the step. 



Although the spine has not a strict resemblance to the mast, the 

 contrivances of the ship-builder, however different from the provis- 

 ions of nature, shows what object is to be attained ; and when we are 

 thus made aware of what is necessary to the security of a column on 

 a moveable base, we are prepared to appreciate the superior provis- 

 ions of nature for giving security to the human spine. 



The human spine rests on what is called the pelvis, or basin ; — a 

 circle of bones, of which the haunches are the extreme lateral parts ; 

 and the sacrum (which is the keystone of the arch) may be felt at the 

 lower part of the back. To this central bone of the arch of the pelvis 

 the spine is connected ; and, taking the similitude of the mast, the sa- 



