STRUCTURE OF THE VERTEBRATE SKELETON. 989 



found to be embedded in its substance, and at later periods, even in the med- 

 ullary cavity. (Duhamel and others. ) 



Most of the smaller bones have but one ossific centre. In the large hip- 

 bones, three primary ossitic centres are formed, one each for the ilium, is- 

 chium, and os pubis ; these grow and finally coalesce around and at the bottom 

 of the acetabulum. In the vertebrae generally, three primary ossific centres 

 appear, and then join around the vertebral ring, the bone being afterwards 

 completed by five epiphyses. In both these instances, and also in the case of 

 the occipital foramen and the cranial cavity, the arrangement described facili- 

 tates the expansion of the cavity or canal around which the bones are destined 

 to grow. The sternum is formed by the coalescence of many pieces. The 

 cartilages of the ribs, and of the nose, are the unossified parts of the precursory 

 costal and nasal cartilages. Sometimes the number of ossific centres has ref- 

 erence to the homological relations of the bone. 



The order in which the ossific process begins in the various bones of the 

 skeleton is very singular, not always coinciding with that in which the carti- 

 laginous rudiments of the bones appear. The clavicles are the first bones to 

 show ossific centres, and then the lower jaw, which has one in each lateral 

 half. Next in order are the vertebrae, the humerus, the femur, the ribs, and 

 the lower and larger portion of the occipital bone. Then, the upper jaw-bones, 

 the frontal bone, the scapula, the radius and ulna, and the tibia and fibula. 

 After that, most of the other cranial and facial bones ; the iliac bones, the 

 metacarpus, the metatarsus, and the phalanges of the fingers and toes ; the 

 ethmoid and turbinated bones, the sternum, the ischium and the os pubis ; 

 the os calcis, and another of the tarsal bones, named the astragulus, and then 

 the hyoid bone. At birth, and for some time afterwards, all the carpal bones, 

 the five smaller tarsal bones, the last pieces of the coccyx, the patella, and the 

 sesamoid bones, are still entirely cartilaginous, having no ossific centres in 

 them. By the end of the fifth year, all these, except the scaphoid, trapezoid, 

 and pisiform carpal bones, are ossifying, the last-named bone not showing any 

 ossific deposit until the twelfth year. The various epiphyses of the long and 

 and other bones are not all finally joined by osseous union to their respective 

 shafts or chief masses, until after the completion of the full period of growth 

 of the body, or about the twentieth or twenty-first year. 



The Vertebrate, Skeleton generally. In examining the skeleton of the Verte- 

 brate series of animals, progressive stages of development, from a cartilaginous 

 to a more and more osseous condition, may be recognized. Low in the scale, 

 as in the Amphioxus, the skeletal framework is composed of a hyaline sub- 

 stance, containing nucleated cells, between which are very fine fibres. In the 

 Myxinoid fishes, it is composed of very distinct fibres, with cartilage cells in- 

 termixed. In the Chimaera, it consists, in some parts, of fibro-cartilage, and, 

 in others, of cartilage. The vertebral column of the sturgeon is a mixture of 

 cartilage, fibro-cartilage, and bone. In the skates and sharks, the cartilaginous 

 skeleton is covered in parts, or entirely, with a crust of ossific matter. In the 

 Lophius, the bones are fibrous and osseous. Lastly, in the so-called Osseous 

 Fishes, the skeleton is entirely bony. 



In the ossified parts of the skeleton of the Cartilaginous Fishes, the bony 

 matter consists either of an irregular granular deposit, between and within 

 the cartilage cells, or of polyhedral bone cells, or of ramified bony laminae. In 

 the less perfectly formed bone, neither lacunee, canaliculi, laminae, nor Haver- 

 sian canals exist. In the more complete bone of the Osseous Fishes, those 

 elements are introduced by degrees. The Haversian canals, in some cases, 

 appear as a few long channels, from which simple canaliculi are given off. In 

 a still higher structure, lacunae, of a peculiar form, are introduced, of mod- 

 erate size, tapering form, and sending out very short wide canaliculi. Fre- 

 quently, the lacunae of different layers of Fishes' bone, cross each other at 

 acute angles ; but more commonly they are arranged in parallel lines. Some- 

 times no Haversian canals exist ; but usually they are present, though small. 

 In rare instances, fine concentric lines are visible around these canals, repre- 

 senting rudimentary laminae. In the lepidosiren, the lacunae are very large, 

 and the canaliculi much branched ; they thus approach the characters of bone 

 in Amphibia. 



