SKELETON 



485 



ossification and investing bones begin with the 

 Ganoid fishes, and are numerous in Teleosteans 

 and in all higher animals. In the development of 

 the individual there is a parallel progress. 

 Theory of the Skull. About the begin- 

 ning of the 19th century Oken and Goethe 

 independently suggested what is known 

 as the vertebral theory of the skull an 

 undoubtedly suggestive theory, to which 

 Owen lent the strength of his authority, 

 but which has been disproved by the sub- 

 Berjuent discoveries of comparative ana- 

 tomy and embryology (see SKULL). 



The Appendicular Skeleton. Somewhat 

 apart from the axial skeleton are the 

 liiiilis and the girdles to which these are 

 attached. No secure conclusion has yet 

 been reached as to origin of the limbs of 

 Vertebrates. In the simplest forms the 

 Tunicate*, the lancelet, the Cyclostomata 

 there are none, and there is a very 

 marked difference between the fin-like 

 limbs of fishes and the fingered and toed 

 limlis which occur in almost all higher 

 backboned animals. According to Gegen- 

 banr, the pectoral and pelvic girdles are 

 structures comparable to the branchial 

 arches, and he supposes that the primi 

 tive limbs were made up of modified fin- 

 rays comparable to those which support 

 the unpaired fins of fishes. According 

 to Dohrn the limbs are residues of a longi- 

 tudinal series of segmentally arranged 

 outgrowths, perhaps comparable to the 

 appendages of a typical Annelid worm 

 (see FISHES, VERTEBRATA). 



The pectoral or shoulder-girdle consists 

 of a dorsal shoulder-blade or scapula, a 

 ventral coracoid, with the articulation for 

 the arm between them, and of a forward- 

 growing collar-ljone or clavicle. The 

 pelvic or hip-girdle consists on each fide 

 of a dorsal ilium, a ventral ischium, with 

 the articulation for the leg between them, 

 and of a third pubic portion. The fore- 

 limb from Amphibians onwards con- 

 sists of a humerus articulating with the 

 girdle, a lower arm composed of radios 

 and ulna lying side by side, a wrist or 

 carpus of several elements, a hand with 

 metacarpal bones in the palm and with 

 fingers composed of several joints or phalanges. 

 The hind-limb from Amphibians onwards con- 

 sists of a femur articulating with the girdle, a 

 lower leg composed of tibia and fibula lying side by 



side, an ankle 

 region or tarsus 

 of several ele- 

 ments, a foot 

 with metatarsal 

 bones in the sole 

 and with toes 

 composed of 

 several joint* or 

 phalanges. Dis- 

 tinct from all the 

 other bones are 

 a few little ' sesa- 

 moids ' which are 

 occasionally de- 

 veloped within 

 tendons and 



skeleton (fig. 5). Altogether there are more thaa 

 200 bones, but some which are originally distinct 

 become fused with their neighbours. 



CERVICAL VERTEBRA 



Fig. 4. 



A, diagram of half of an Ideal pectoral 

 ginlle : a, clavicle ; b, ocapula ; c, cor- 

 acoid. B, diagram of half of the pelvic 

 girdle of an alligator : d, ilium ; e, 

 uchlnm ; /, pubin. 



near joints, notably, for instance, the knee-pan or 

 patella. 



The Skeleton of Man. As the bones of all the 

 chief parts of the human body are described in 

 separate articles, we need not do more than unify 

 these by reference to a diagram of the entire 



Kg. 5. Human Skeleton. 



In the vertebral column there are originally thirty- 

 three vertebra^, but in adult life the normal number 

 is twenty-six, for, while the first twenty -four remain 

 distinct, five (the twenty-fifth to the twenty-ninth 

 inclusive) unite to form the sacrum supporting the 

 hip-girdle, and the four hindmost fuse more or less 

 completely in a terminal tail-piece or coccyx. Seven 

 cervicals support the neck ; twelve dorsals form the 

 greater part of the back and bear ribs ; five lumbars 

 occur in the loins ; these are followed by the sacrum 

 and the coccyx (see SPINAL COLUMN). 



The rilis, or elastic arches of bone which bound 

 the chest, are normally twelve on each side. Most 

 of them articulate dorsally with the bodies of two 

 adjacent vertebrae and with the transverse pro- 

 cesses of the posterior one ; ventrally the first seven 

 pairs are connected with the median breastbone by 

 means of intervening cartilages, while the posterior 

 five pairs are more or less free (see RIBS). 



The skull consists in early adult life of twenty- 

 two separate bones, but originally there were more, 

 and as life continues the nnmlier may be further 

 reduced by fusion. For the various bones, see 

 SKULL. 



The skeleton of the arm includes thirty bones in 

 the upper arm the hnmerus, which articulates with 

 the shoulder-girdle ; in the forearm the radius and 



