MECHANICS, (ANIMAL.) 



747 



city of the substance is many times more tlmn sufficient 

 to resist gravity, and therefore aids the form to resist 

 forces of other kinds, operating in all directions. 

 When we reflect on the strength displayed by the 

 arched film of an egg-shell, we need not wonder at 

 the severity of blows which the cranium can with- 

 stand. 



Through early childhood, the cranium remains, to 

 a certain degree, yielding and elastic ; and the falls 

 and blows so frequent during the lessons of walking, 

 &c., are borne with impunity. The mature skull 

 consists of two layers or tables, with a soft diploe 

 between them, the outer table being very tough, with 

 its parts dovetailed into each other, as tough wood 

 would be by human artificers ; while the inner table 

 is harder, and more brittle (hence called vitreous], 

 with its edges merely lying in contact, because its 

 brittleness would render dovetailing useless. 



A very severe partial blow on the skull generally 

 fractures and depresses the part, as a pistol bullet 

 would ; while one less severe, but with more extended 

 contact, being slowly resisted by the arched form, 

 often injures the skull by what is correspondent to 

 the horizontal thrust in a bridge, and causes a crack 

 at a distance from the place struck, generally half 

 way round to the opposite side. Sometimes, in a fall 

 with the head foremost, the skull would escape 

 injury, but for the body, which falls upon it, pressing 

 the end of the spine against its base. 



In the lower jaw, we have to remark the greater 

 mechanical advantage, or lever power, with which 

 the muscles act, than in most other parts of animals. 

 The temporal and masseter muscles pull almost 

 directly, or at right angles to the line of the jaw; 

 while in most other cases, as in that of the deltoid 

 muscle lifting the arm, the muscles act very obliquely, 

 and with power diminished in proportion to the 

 obliquity. An object placed between the back teeth 

 is compressed with the whole direct power of the 

 strong muscles of the jaw ; hence the human jaw can 

 crush a body which offers great resistance, and the 

 jaws of the lion, tiger, shark, and crocodile, &c., are 

 stronger still. 



The teeth rank high among those parts of the 

 animal body which appear almost as if they were 

 severally the fruits of distinct miraculous agencies, 

 so difficult is it to suppose a few simple laws of life 

 capable of producing the variety of form so beauti- 

 fully adapted to purposes which they exhibit. They 

 constitute an extraordinary set of chisels and wedges, 

 so arranged as to be most efficient for cutting and 

 tearing the food, and, with their exterior enamel, so 

 Lard that, in early states of society, teeth were made to 

 answer many purposes for which steel is now used. 

 It seems, however, as if the laws of life, astonishing 

 as they are, had still been inadequate to cause teeth, 

 cased in their hard enamel, to grow as the softer 

 bones grow ; and hence lias arisen a provision more 

 extraordinary still. A set of small teeth appear soon 

 after birth, and serve the child until six or seven years 

 of age: these then fall out, and are replaced by larger 

 ones, which endure for life ; the number being com- 

 pleted only when the man or woman is full grown, 

 by four teeth, called wisdom teeth, because they 

 come so late, which rise to fill up the then spacious 

 jaw. 



The spine, or back-bone, has, in its structure, as 

 much of beautiful and varied mechanism as any single 

 part of our wonderful frame. It is the central pillar 

 of support, or great connecting chain of all the other 

 parts ; and it has, at the same time the office of con- 

 taining within itself, and of protecting from external 

 injury, a prolongation of the brain, called the spinal 

 marrow, more important to animal life than the 

 greater part of the brain itself. We shall see the 



spine uniting the apparent incompatibilities of great 

 elasticity, great flexibility in all directions, and great 

 strength, both to support a load and to defend its 

 important contents. 



Elasticity. The head may be said to rest on the 

 elastic column of the spine, as the body of a carriage 

 rests upon its springs. Between each two of the 

 twenty-four vertebrae, or distinct bones, of which the 

 spine consists, there is a soft elastic intervertebral 

 substance, about half as bulky as a vertebra, yielding 

 readily to any sudden jar ; and the spine, moreover, is 

 waved, or bent a little, like an italic f, as seen when it 

 is viewed sideways ; and, for this reason, also, it yields 

 to any sudden pressure operating from either end. 

 The bending might seem a defect in a column intended 

 to support weight ; but the disposition of the muscles 

 around is such as to leave all the elasticity of the 

 bend and a roomy thorax, without any diminution of 

 strength. 



Flexibility. The spine may be compared to a 

 chain, because it consists of twenty-four distinct 

 pieces, joined by smooth rubbing surfaces, so as to 

 allow of motion in all directions ; and a little motion, 

 comparatively, between each two adjoining pieces, 

 becomes a great extent of motion in the whole line. 

 The articulating surfaces are so many, and so exactly 

 fitted to each other, and are connected by stirli 

 number and strength of ligaments, that the combina- 

 tion of pieces is really a stronger column than a 

 single bone of the same size would be. 



The strength of the spine, as a whole, is shown in 

 a man's easily carrying upon his head a weight 

 heavier than himself, while each separate vertebra 

 is a strong irregular ring, or double arch, surround- 

 ing the spinal marrow. The spine increases in size 

 towards the bottom, in the justest proportion, as it 

 has more weight to bear. 



The Ribs. Attached to twelve vertebras, in the 

 middle of the back, are the ribs, or bony stretchers 

 of the cavity of the chest, constituting a structure 

 which solves, in the most perfect manner, the difficult 

 mechanical problem of making a cavity with solid 

 exterior, which shall yet be capable of dialing and 

 contracting itself. Each pair of corresponding ribs 

 may be considered as forming a hoop, which hangs 

 obliquely down from the place of attachment behind; 

 and so that, when the fore part of all the hoops is 

 lifted by the muscles, the cavity of the chest is 

 enlarged. 



We have to remark the double connexion of the 

 rib behind, first to the. bodies of two adjoining verte- 

 bras, and then to a process or projection from the 

 lower, thus effecting a very steady joint, and yet 

 leaving the necessary freedom of motion ; and we 

 see the fore part of the rib to be of flexible cartilage, 

 which allows the degree of motion required there, 

 without the complexity of a joint, and admirably 

 guards, by its elasticity, against the effects of sudden 

 blows or shocks. 



The muscles which have their origin on the ribs, 

 and their insertion into the bones of the arm, afford 

 us an example of action, and reaction being equal 

 and contrary. When the ribs are fixed, these 

 muscles move the arm ; and, when the arm is fixed 

 by resting on a chair or other object, they move the 

 ribs. This is seen in fits of asthma and dyspnea. 



The shoulder-joint is remarkable for combining 

 great extent of motion with great strength. The 

 round head of the shoulder-bone rests upon a shallow 

 cavity in the shoulder-blade, that it may turn freely 

 in all ways ; and the danger of dislocation from tins 

 shallowness is guarded against by two strong bony 

 projections above and behind. To increase the 

 range of motion to the greatest possible degree, the 

 bone called the shoulder-blade, which contains the 



