Keill has observed, that the breast-bone, in an easy inspiration, is thrust out one-tenth of an inch: and he calculates that this, added to what is gained to the space within the chest by the flattening or descent of the diaphragm, leaves room for forty-two cubic inches of air to enter at every drawing-in of the breath. When there is a necessity for a deeper and more laborious inspiration, the enlargement of the capacity of the chest may be so increased by effort, as that the lungs may be distended with seventy or a hundred such cubic inches. The thorax, says Schelhammer, forms a kind of bellows, such as never have been, nor probably will be made by any artificer.* of the chest by elevating the ribs; and, during the momentary interval of muscular action, the cartilages, from their great elasticity, restore the ribs to their former position. Dr. Thompson considers the ordinary quantity of air contained in * V. The patella, or knee-pan, is a curious little bone; in its form and office, unlike any other bone of the body. It is circular; the size of a crown piece; pretty thick; a little convex on both sides, and covered with a smooth cartilage. It lies upon the front of the knee and the powerful tendons, by which the leg is brought forward, pass into it, (or rather it makes a part of their continuation,) from their origin in the thigh to their insertion in the tibia.t It protects both the tendon and the joint from any injury which either might suffer by the rubbing of one against the other, or by the pressure of unequal sur faces. It also gives to the tendons a very considerable the lungs is 280 cubic inches, and that there enters into, or goes out, at each inspiration or expiration forty inches. Thus, supposing twenty inspirations in a minute, the quantity of air which would enter and pass out in this time would be 800 inches; which makes 48,000 in the hour, and in twenty-four hours 1,152,000 cubic inches. *Fig. 2. Represents the true shape of the patella, the anterior surface convex. Fig. 3, the posterior surface, which has two concave depressions adapted to the condyles of the thigh bone. The projection of the patella, as a lever, or pulley, removes the acting force from the centre of motion, by which means the muscles have a greater advantage in extending the leg. That this bone is "unlike any other in the body," is a mistake; such bones are numerous, though less obvious, for they do not exceed the size of a pea these are called sesamoid bones, and are formed in the flexor tendons of the thumb, and sometimes in the fingers. They are frequently found under the tendons of some of the muscles. Two of these sort of bones are constantly found under the articulation of the great toe with the foot some also are discovered, though not so constantly, under the corresponding joints of the other toes. The sesamoid bones, like the patella, remove their tendons from the centre of motion, facilitate their glidings over the bone, and protect their articulations. : : In this figure, b, c, d show the ligaments of the patella. mechanical advantage, by altering the line of their direction, and by advancing it farther out from the centre of motion; and this upon the principles of the resolution of force, upon which principles all machinery is founded. These are its uses. But what is most observable in it is, that it appears to be supplemental, as it were, to the frame; added, as it should almost seem, afterward; not quite necessary, but very convenient. It is separate from the other bones; that is, it is not connected with any other bones by the common mode of union. It is soft, or hardly formed, in infancy; and produced by an ossification, of the inception or progress of which no account can be given from the structure or exercise of the part. VI. The shoulder-blade* is, in some material respects, a very singular bone; appearing to be made so expressly for its own purpose, and so independently of every other reason. In such quadrupeds as have no collar-bones, which are by far the greater number, the shoulder-blade has no bony communication with the trunk, either by a joint, or process, or in any other way. It does not grow to, or out of, any other bone of the trunk. It does not apply to any other bone of the trunk; (I know not whether this be true of any second bone in the body, except perhaps the os hyoides :†) in strictness, it forms no part of the skeleton. It is bedded in the flesh; attached only to the muscles. It Fig. 4. This bone (the scapula) is joined to the collar-bone by ligaments, and to the thorax by powerful muscles which are capable of sustaining immense weights, and whose action gives the various directions to the arm, and enables it freely to revolve at the shoulder-joint. The os hyoides is a small bone situated at the root of the tongue, Fig. 5. It serves as a lever or point for attaching the muscles of the tongue, larynx, and those of deglutition. The figure is introduced, as it will be again referred to. |