Page:The New International Encyclopædia 1st ed. v. 18.djvu/255

* SKELETON. 211 SKELETON. each somite, the tilth or unpaired piece form- iuj; the dorsal spinous process. Ventral car- tilaginous pieces also occur ventral to the chorda. The bodies (centra) of the vertebra' definitely appear, and the chorda becomes constricted in- travertebrally, giving the vertebra- an hour-glass iorm. The rings of cartilage formed by intra- vertebral constrictions are biconcave or "amphi- ciclous" in all fishes with bony vertebrie and in most Urodela ; also in a few fossil and living rep- tiles (q.v.), and iu a few fossil birds. So long as the separate vertebrcc of the vertebral column are amphicoelous their connection with one an- other must depend upon something else than the bony vertebrie themselves. In the lower fishes this union is efTected by the chorda and chordal sheath. In the lower Urodela it is efTected by the intervertebral, non-ossified cartilage. In the higher Urodela, the Anura and almost all the reptiles, however, the vertebrae are linked to- gether by means of a ball-and-socket joint. The concavity may be on the posterior and the con- vexity on the anterior end (opisthocfflous) or conversely { proccelous ) . In crocodiles, birds, and mammals the opposed faces of the vertebra; are ajiproximately plane surfaces. In the develop- ment of the vertebra- of man the phylogenetic stages are recapitulated. Tlie typical vertebra of man consists of a centrum from which an arch arises dorsally to protect the spinal cord. These arches together constitute the neural canal. Each half arch is composed of the rounded pedicle and the broad flat lamina. There are three kinds of processes: (1) the dorsal or neuropophysis: (2) the transverse process, serving for the attach- ment of the muscles which keep the vertebrie to- gether ; (3) the forward and backward articulat- ing processes ( zj'gopophyses ) . The relation of the centra to the somites of the body is an in- teresting one. They do not arise one in the mid- dle of each .somite, but at the plane of separation of adjacent somites, thus insuring flexibility in the column. The number of vertebrie in mammals is highly variable in difi'erent species. With one or two exceptions all mammals have seven cervical (non- rib-bearing) vertebr.-e. All the artiodactyls pos- sess nineteen thoraeico-hmibar vertebrse. The smallest number (fourteen) occurs in arma- dillos: the largest (thirty) in hyraeoids. Since the number of vertebrii; corresponds to that of the somites of the body, it seems necessary to conclude that the latter are highly vari- able in number. If we seek for an inter- pretation of the differences in the vertebral column we may find it in the difi'erent tasks tlie parts perform, and the difl'erentiation of vertebrie is a late acquisition, gradually ac- quired with advancing age. The sacral bones begin their fusion only at sixteen years, and this is not completed until the age of thirty. The sacrum is composed of four or five caudal verte- brie fused together. Ribs. Ribs are also a part of the axial skele- ton. Ventrally they end in cartilage and dorsal- ly in two articular surfaces. The main part of the bone is the 'shaft' or 'body.' and its dorsal articular surface the 'head:' on the side near the head is a second articular surface, the 'tube- rosity:' between this and the head there is a con- striction, the 'neck.' In man the last of the nor- mally twelve ribs is occasionally reduced to an in- significant rudiment, or a thirteenth rib may be present. The transverse process of the seventh cervical vertebra and that of the first thoracic are quite dill'erent. The ventral arm of the trans- verse process of the seventh vertebra represents the rib. Similarly it niiiy be inferred, even from the adult conditions, that all the cervical and trunk vertebrie possess ribs or the rudiments of ribs; and embryologj' bears out this conclu- sion. Sternum. The sternum or 'breastbone' of man is a flat bone to which the ventral ends of the ribs are attached. Its anterior part is known as the 'manubrium,' the middle part as the 'gladi- olus,' and the posterior cartilaginous tip as the '.xiphoid' or 'ensiform appendi.x.' The middle p;irt is composed of more than one piece. In nearly all the lower mammals it is made up of as many bones as there are pairs of ribs attached to it, and this composition may be plainly seen in the sternum of a child. Moreover, the sternum of the joung of many mammals shows a double origin, and it is plain that the sternum, if a product of the fusion of the ventral ends of the tiioracic ribs, was originally laid down as a paired structure. The sternum of lower vertebrates is often closely vmited to the shoulder-girdle and possesses an accessory bone — the episternum. The sternum of Amphibia is small and the ribs do not meet ventrally. The sternum of most earinate birds is strongly keeled to permit of the attach- ment of powerful muscles of flight. See Bird. Skull. We may distinguish in the skull the cranium or braincase and the visceral skeleton. In the development of the human skull three stages may be distinguished which correspond with phylogenetic stages: (1) The fibro-connec- tive tissue stage. This is represented in phy- logeny by the condition in Amphioxus, where a fibrous cordal sheath surrounds the notochord. (2) The cartilaginous stage. In the anterior region of adult selachians a large cartilaginous capsule, open above, completely surrounds the brain below and laterally, derived from two pairs of cartilage plates. Ventral to the skull the visceral skeleton arises, consisting of the upper and lower jaws and the six branchial arches, the foremost of which early diflferentiated itself from the other five, entered into connection with the lower jaw, and constitutes the hyoid arch. The lower jaw arises in a manner ])recisely equivalent Key to Skeleton Plate. 1. Frontal bone. 16. .Sacrum. ■2_ Parietal bone. 17. Head of femur. 3.' Temporal bone. 18. .Shaft of femur. 4. Occipital bone. 19. Patella. 5. Malar bone. •20. Sliaft of tibia. 6. Superior maxillar.v. 21. Fibula. 7. Inferior ninxillarj. •22. Greater trochanter of R. Cervioa! vertebra?. femur. 9. NaHal bone. 23. Condyles of femur. 10. Sternum. 24. Tuberosity of tibia. 11. Humerus. 26. Clavicle. 12. Ulna. 26. Condyles of humerus. 13. Radius. 27. Head of radius. 14. Lumbar vertebrae. 2,S. Dorsal vertebrie. 15. Innominate bones. 29. Hand. Scapula. 3. Pbalanges. e. Unciform. h. Metacarpals. t Trapezoid. c. Trapezium. p. Pisiform. d. Scaphoid. Foot. h. Astragalus. m. Cuboid. i. Calcaneum. B. Navicular. ./*. Metatarsus. o. Ectocuneiforw, A'. I'iialanges. /). Mesoeiineiform. 1. Entoeuneilorm.