VI. THE ENDOSKELETON: THE COMPARATIVE ANATOMY OF 

 THE VERTEBRAL COLUMN AND RIBS 



A. GENERAL CONSIDERATIONS ON THE ENDOSKELETON 



i. The parts of the endoskeleton. — We have already defined the endoskeleton as the 

 internal skeleton of the body. The notochord is the first endoskeleton of the chordates and 

 the principal endoskeleton of the lower chordates. In the vertebrates the notochord is always 

 more or less replaced by a skull and vertebral column, and the vertebrates possess in addition 

 other components of the endoskeleton in association with the gills and paired appendages. 

 The parts of the endoskeleton of vertebrates are: the skull in the head; the visceral skeleton 

 composed of gill arches supporting the gills; the vertebral column occupying the median dorsal 

 region; the ribs, projecting from the vertebrae, one pair to each vertebra primitively; the 

 sternum, occupying the median ventral region of the anterior part of the trunk; the pectoral 

 girdle supporting the anterior paired appendages; the pelvic girdle, supporting the posterior 

 paired appendages; and the skeleton of the appendages. The four parts first named con- 

 stitute the axial skeleton, while the other parts constitute the appendicular skeleton. 



2. The skeletogenous regions. — The endoskeleton develops from mesenchyme. The 

 mesenchyme accumulates in certain regions known as the skeletogenous regions where skeleton 

 is to be formed. The arrangement of these skeletogenous regions is to some extent depend- 

 ent on the disposition of the myotomes. As we have already learned, the myotomes or muscle 

 segments, which consist of those portions of the epimeres remaining after the formation of 

 mesenchyme, grow down between the skin and the digestive tract, so as to form the muscular 

 layer of the body wall. (Review Fig. 10, p. 43.) Each myotome is separated from the adjacent 

 ones by a transverse partition or plate of mesenchyme, called the myoseptum or myocomma. 

 Each myotome is further divided into a dorsal and a ventral half by a horizontal partition, 

 the horizontal skeletogenous septum, which extends from the notochord to the level of the 

 lateral line on the sides of the body. The notochord and neural tube are also surrounded by 

 mesenchyme which extends from the neural tube to the median dorsal line, forming the 

 dorsal skeletogenous septum, and from the notochord to the median ventral line (in the tail) 

 forming the ventral skeletogenous septum. In the trunk region the ventral skeletogenous 

 septum is naturally split into two septa, ventrolaterally situated, on account of the inter- 

 vention of the coelom. The horizontal, dorsal, and ventral septa are, it is to be understood, 

 continuous longitudinal septa, extending the length of the body. The skeletogenous septa 

 are illustrated in Figure 15, also in K, Figure 33, page4r, andW, Figure S3, P a ge 125. As their 

 name implies, the skeletogenous septa are regions of skeleton formation. At the intersection of 

 every myoseptum with the medially placed mesenchyme of the dorsal and ventral septa and that 

 surrounding the notochord and neural tube a vertebra arises. As the myosepta are segmentary 

 repeated, owing to the primary segmentation of the myotomes, it follows that the vertebrae 

 are also segmentally arranged and that the vertebrae alternate with the myotomes. 



3. Cartilage and membrane bones. — The mesenchyme in the process of forming endo- 

 skeleton first produces cartilage. All of the endoskeleton proper is first composed of cartilage, 

 and in the lower vertebrates the endoskeleton may remain wholly or partly cartilaginous. 

 The endoskeleton of elasmobranchs, for example, is composed entirely of cartilage. Such 

 cartilage may be and often is stiffened by the deposition within it of calcium salts. In such 

 cases the cartilage is said to be calcified. In most vertebrates, however, the cartilage is more 



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