v BIBS 303 



structures, which may be distinguished by the names dorsal or upper 

 ribs and ventral or lower ribs. 



In Polypterus both sets of ribs are well developed — the dorsal 

 ones larger towards the head, the ventral larger towards the tail. 

 In other vertebrates the rule is that only one set is developed, 

 though the other may be represented by more or less distinct rudi- 

 ments or vestiges. Thus in Actinopterygian ganoids, Teleosts and 

 Dipnoans the ribs are ventral ribs while in Elasmobranchs, Amphibians 

 and Amniotes they are dorsal ribs. 1 



Both types are associated with the myosepta hut whereas the 

 dorsal ribs lie at the level of the horizontal septum which divides 

 the lateral musculature into a dorsal and a ventral half, the ventral 

 ribs, on the other hand, lie along the peritoneal edge of the myo- 

 septum where it abuts on the lining of the splanchnocoele. 



Probably both sets of ribs are to be interpreted morphologically 

 as outgrowths from the vertebrae and the balance of evidence appears 

 to favour the view that both are fundamentally outgrowths from the 

 series of haemal arch-elements. 



Ventral Ribs. — This is clearly the case with the ventral ribs 

 which are simply the ventral prolongations of the haemal arch- 

 elements, frequently jointed off from the basal stump of the arch 

 (transverse process) by the conversion of a thin layer of the cartilage 

 into fibrillar material. In the skeleton of a Lung -fish, a Crosso- 

 pterygian, or an Actinopterygian the ribs are seen to form a perfectly 

 continuous series with the haemal arches of the tail region. 



Dorsal Ribs. — The nature of the dorsal ribs tends to be obscured 

 by the fact that their point of attachment to the vertebra shows 

 much variation e.g. they may appear to arise not from the haemal 

 but from the neural arch. That we have to do here with a secondary 

 shifting in a dorsal direction is indicated by various considerations. 

 Amongst the Bays it can sometimes be seen that the ribs towards 

 the head end of the series become more and more displaced dorsally, 

 so that they come to project from the neural arch. Then it will 

 be remembered that in various fishes the haemal arch -element 

 becomes divided into a ventral part (haemal process) and a dorsal 

 part which latter carries the rib and may undergo a considerable 

 displacement in a dorsal direction. 



In Urodele Amphibians Goeppert has shown that the apparent 

 attachment of the rib to the neural arch has come about in a some- 

 what complicated fashion as illustrated by Pig. 153. The most 

 nearly primitive condition is that shown in the larva of such a 

 perennibranchiate form as Nectwrus (Fig. 153, A). Here the haemal 

 arch - element (h.a) sends off a strong outgrowth (r.b), the " rib- 

 bearer," which passes in a dorsal direction closely applied to the 



1 Distinct traces of dorsal ribs occur in various Teleosts, e.g. Salmonids and 

 Clupeids. The numerous little bones found in the myosepta of various Teleosts in 

 addition to the true ribs are probably to be looked on as independent and secondarily 

 developed "tendon bones," 



