62 



LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



and ventrally, respectively, into neural and haemal spines. In addition the centrum commonly 

 presents various projecting processes, known as apophyses, serving for articulations with adjoin- 

 ing vertebrae or with ribs or for the attachment of muscles. In lower forms there are not 

 uncommonly two centra or arches, one anterior and one posterior, to each metamere, but 

 usually only one of these structures is present to each segment, owing to the loss or fusion of 

 the others. Figure 19 illustrates a typical vertebra with its parts. 



5. The origin of ribs.— Each vertebra is theoretically provided with a pair of ribs which 

 project out from the centrum into the body wall and serve to strengthen the latter. The 

 ribs, like other parts of the axial skeleton, arise in the skeletogenous septa. There are two 

 kinds of ribs, both of which arise in the myosepta and consequently are segmental in arrange- 

 ment. One type of rib is formed at the intersection of each myoseptum with the horizontal 

 skeletogenous septum. Since the horizontal septum divides the myotomes into dorsal and ven- 

 tral halves (see Fig. 15^4, p. 58) such ribs lie between muscles and hence are named intermus- 

 cular ribs. They are also known as dorsal or true ribs. Intermuscular ribs are characteristic of 



the majority of vertebrates. The 



neural spine 



second type of rib arises at the 

 points of intersection of the myo- 

 septa with the ventral skeletoge- 

 nous septum or its derivatives. It 

 will be recalled that in the trunk 

 region the ventral septum is split 

 into two lateral septa, owing to 

 the intervention of the coelom and 

 its contents on the ventral side of 

 the body (see Fig. 1 5B) . The ribs 

 under consideration appear at the 

 points of intersection of the myo- 

 septa with these ventrolateral 

 septa. They lie just outside the 

 coelomic lining, between the coe- 

 lomic wall and the muscle layer. 

 They are designated as subperi- 

 toneal ribs; also as ventral or false 

 ribs. 1 Subperitoneal ribs are char- 

 acteristic of the bony fishes. Both 

 types of ribs may be regarded as 



extensions of the basiventral arcualia, their position in the myoseptum being shifted dorsally 



or ventrally in different vertebrates. In many fishes both kinds of ribs are simultaneously 



present on each vertebra. The two kinds of ribs and their morphological relations to the 



body wall are illustrated in Figure 20. 



Ribs, like vertebrae, appear first as collections of mesenchyme which later transforms 



into cartilage. The ribs are permanently cartilaginous in some fishes but in other vertebrates 



are ossified, at least in part. 



References on the vertebral column and ribs are: K, pages 4S-57, 60-63 1 ^ ', pages 122- 



38; Wd, pages 44-70; P and H, pages 72-75; L, pages 98-101; CNH, Vol. VII, pages 193-206. 



1 The term pleural as applied to ribs is here avoided, owing to the fact that it is differently employed 

 in different texts. K and W use the term pleural as synonymous with intermuscular, while L uses it as 

 synonymous with subperitoneal. The latter usage appears to be correct. 



neural tube 

 neural arch 

 of vertebra 

 centrum of 

 vertebra 



epaxial muscles 



notochord 



horizontal skeletogenous 

 septum 



intermuscular rib 



subperitoneal rib 



hypaxial muscles 



-dorsal mesentery 



coelom 



intestine 



peritoneum 



Fig. 20. — Diagrammatic cross-section through the trunk 

 of a vertebrate showing the relation of the ribs to the skele- 

 togenous regions and the positions of the two kinds of ribs. 

 (From Parker and Haswell's Textbook of Zoology, courtesy of 

 the Macmillan Company.) 



