I04 HUMAN ANATOMY. 



central, many-jointed rod bent forward near the top, with very perfect bony walls 

 behind and above it, enclosing the central nervous system, and very imperfect bony 

 and cartilaginous walls before and below it, enclosing the digestive apparatus and its 

 associates, the circulatory, respiratory, urinary, and reproductive organs. 



The Appendicular Skeleton has an entirely distinct origin ; it is the frame- 

 work of the limbs. It consists of two girdles, a thoracic and a pelvic, to each of 

 which is attached a series of segments, the terminal one of which expands into five 

 rays, — -fingers and toes. According to some anatomists, the true vertebrate plan is 

 of seven terminal rays, but, the question being still undecided, the more usual sys- 

 tem is followed. Each of these rays consists of three or four bones. Proximal to this 

 comes a series of short bones, — wrist and ankle ; still nearer, a pair of bones, — -fore- 

 arm and leg ; then a single bone, — arm and thigh,; and lastly a bony arch, — the 

 girdle. 



In man, the thoracic girdle, made up of collar-bone and shoulder-blade, lies 

 external to the chest, while the pelvic girdle fuses on each side into one bone, meets 

 its fellow in front, and unites with the bodies of certain vertebrae. Thus, besides 

 bearing a limb, the pelvic girdle forms a part of the wall of the abdominal and the 

 pelvic cavities and would seem to belong to the axial skeleton, but embryology and 

 comparative anatomy show that it does not. 



GENERAL CONSIDERATION OF THE BONES. 



The bones have the physiological function of bearing weight, of affording pro- 

 tection, and especially, by the systems of levers composing the limbs, of effecting 

 movements through the action of the muscles. They must, therefore, be capable of 

 resisting pressure, accidental violence, and the strain caused by the pull of the 

 muscles. The size of the bones must be such that besides serving the obvious needs 

 of support and protection they may be suiftciently large to of5er adequate surface for 

 the origin and insertion of muscles, and the shape must be such as to allow this 

 without undue weight. 



Shapes of Bones. — Bones are divided, according to their form, into long, 

 fiat, and irregular ; such classification, however, is of little value, since many bones 

 might be variously placed. 



Long bones form the best-defined group. They consist of a shaft and two 

 extremities, each of which takes part in the formation of a joint, or, as in the case of 

 the last phalanges, is terminal. 



Flat bones, where very thin, consist of a single plate ; where thicker, they con- 

 sist of two plates separated by spongy substance called diploe. 



Irregular bones may be regarded as embracing all others. The group of the 

 so-called short bojies has no significance. 



Sesamoid Bones, with the exception of the patella, are not usually included 

 in the description of the skeleton. With the above exception, they are small rounded 

 bones developed, for the most part, in the capsules of joints, but sometimes in ten- 

 dons. Usually one surface is cartilage-covered, and either enters into the formation 

 of a joint or, separated by a bursa, plays on another bone, or on cartilage or liga- 

 ment. Their function is to obviate friction, and, in some cases, to change the direc- 

 tion of the pull of a muscle. The number of sesamoid bones is very variable ; but 

 the usual idea that they are, so to speak, accidental, depending on the mechanics of 

 a certain joint or tendon, must probably be abandoned. They are rather to be con- 

 sidered as real parts of the skeleton,' all of which have their places in certain 

 animals, but all of which either are not developed, or, if they do appear, are again 

 lost in others. Thus, certain sesamoid bones of the fingers are very frequent in the 

 foetus and veiy rare in the adult. ** 



Growth of Bones. — The microscopical details of bone-growth are given else- 

 where (page 94). Sufifice it to say here that each bone has certain so-called centres 

 of ossification from which the formation of the new bone spreads. In the long bones 

 there is one main centre in the shaft, or diaphysis, which appears in the first half of 

 tcetal life. Other centres appear, usually some time after birth, in the ends of the 



' Thilenius : Morpholog. Arbeiten, Bd. vi., 1896. 



