454 CHORDATA 



Since the large body cavity with its viscera is here, the haemapophyses 

 extend outwards and downwards and are divided into two parts, a basal 

 apophysis and a lower movable portion, the rib (fig. 512). Also the union 

 of htemapophyses with haemal spine does not occur; the ribs are either 

 free (fishes) or are (at least in part) connected ventrally by a breast bone 

 or sternum (amniotes, fig. 514). The sternum is a derivative of the ribs. 

 In development the ventral ends of the ribs of a side fuse and then these 

 fused tracts of the two sides unite to form the sternum. 



The hzemal arches lie medial to the longitudinal muscles of the body, and in 

 the trunk region they lie in the same position just beneath the peritoneum. 

 These are hcemal ribs and are found only in teleosts and ganoids. The ribs of 

 all other vertebrates are morphologically different and are called lateral ribs, 

 They develop in a horizontal connective-tissue septum which extends out through 

 the longitudinal muscles from the axial skeleton to the skin, dividing the muscu- 

 lature into dorsal (epaxiat) and ventral (hypaxial) portions (fig. 92). In the 

 elasmobranchs these lateral ribs are attached to the haemapophyses, in the others 

 to diapuphyses, which arise from the neuropophyses, and parapophyses, which 

 arise from the vertebral centres. In the caudal region, often also in the cervical, 

 lumbar, and sacral regions, the lateral ribs and dia- and parapophyses fuse to 

 form transverse processes. These occur together with haemal arches in the 

 tails of many Amphibia and reptiles and some mammals, the haemal arches 

 forming the chevron bones which, as in fishes, enclose the caudal blood-vessels. 

 The presence of intercalaria in cyclostomes, sharks, and ganoids indicates that 

 primitively a double vertebra arose in each somite. Paleontological and 

 embryological researches on reptiles support this view. 



In most vertebrates either the basal ends of the arches broaden out 

 around the notochord and fuse with one another, or perichordal cartilages 

 arise independently, furnishing in either case firm supports, the vertebral 

 bodies, or centra, for the system of arches. These increase in size at the 

 expense of the notochord on the inside, sometimes leading to its almost 

 complete obliteration, as in the mammals; in others, as the fishes, the 

 reduction is less complete. The fishes have amphiccele vertebrae (fig. 513), 

 that is, the centra are hollow at either end. In these cups the notochord 

 exists even in the adult, and when small connecting portions extend 

 through the centra the notochord takes the form of a. rosary with alter- 

 nating enlargements and contractions. 



Histologically the vertebral column may be either cartilage or bone; 

 usually it is first formed in cartilage, which later may be replaced by bone. 

 If the ossification do not occur, the column remains cartilaginous; if 

 incomplete, cartilage and bone appear together. Since these histological 

 differences are combined with varying degrees of persistence of the noto- 

 chord and with modifications in the form of the vertebrae and their pro- 

 cesses, there results an extraordinary variety in the appearance of the 

 vertebral column. 



