VERTEBRA AND RIBS 



23 



Ribs 



Ribs and their supporting structures are closely related to the 

 vertebra?. In considering them it should be recalled that trunk and 

 tail muscles are separated by a horizontal septum (p. 3) into 

 epaxial and hypaxial series, the septum reaching the vertebral 

 column at about the level of the centra. A second sheet of skeleto- 

 genous tissue, the median septum, lies on the medial side of these 

 muscles — between them and the vertebrae in the tail, just outside 

 of the peritoneum in the trunk. These two septa are intersected 

 between the myotomes by the 

 myosepta, and at any intersection 

 skeletal parts may form. 



In the caudal region of fishes 

 the hgemapophyses are developed 

 at the intersection of median and 

 myosepta, and, with the haemal 

 spine, form arches which are usu- 

 ally closely connected with the 

 vertebral centra and enclose the 

 caudal artery and vein (fig. 22, A). 

 Thus the haemapophyses lie on the 

 medial side of the caudal hypaxial 

 muscles. In the trunk of fishes, 

 besides the dorsal aorta, various 

 viscera lie ventral to the vertebrae, 

 some of them (alimentary canal, 

 gonads) liable to great variations in size at different times. Hence 

 the closed haemal arch of the tail is impossible here. But immedi- 

 ately beneath the peritoneum and at the intersection of myosepta 

 and the scleroblastic tissue underlying the ccelomic fining (on the 

 medial side of the hypaxial muscles) there are usually elongated 

 skeletal elements, preformed in cartilage, resembling haemapophyses 

 in every respect, save that they are not connected ventrally by a 

 haemal spine (fig. 22, B. hr). They may be in actual continuity with 

 the centra or may be articulated to a 'basal stump' on the centrum. 

 This condition permits changes in the size of the abdominal cavity. 

 These rods are the ' ribs ' of most fishes, which, from their evident 

 homology with the haemapophyses in the tail, are called haemal ribs. 



Fig. 22. — Relations of muscles and the 

 structures commonly called ribs. A, the 

 condition in the tail of Urodeles; removal 

 of the transverse processes would give the 

 piscine condition; B, shows both kinds of 

 ribs in the trunk; they coexist in but few 

 forms, ep, epaxial muscles; H, hypaxial 

 muscles; h, horizontal septum; ha, hsmal 

 arch; hr, haemal rib; pr, pleural rib; /, 

 transverse process (true rib); va, vertebral 

 artery. 



