RIBS 63 



Manis macrura (about fifty), and the caudal region is most reduced 

 in the higher Primates, in which it forms a stump-like coccyx 

 consisting of at most five to six vestigial vertebrae, all fused 

 together, and these may even (e.g. in Man) unite with the sacrum. 

 In the human embryo of 4-6 mm. in length, a distinct tail 

 is present, consisting of all the characteristic parts ; it gradually 

 undergoes reduction, and what is left no longer projects 

 externally. 



II. RIBS. 



Some doubt still exists as to whether the ribs are to be 

 considered as primitively independent skeletal structures, arising 

 in the intermuscular septa or myocommas, or as parts of certain 

 processes of the vertebrae which have become segmented off from 

 the latter, as is plainly seen to be the case, for example, in embryos 

 of Hatteria. Their relations to the axial skeleton, whether 

 primary or secondary, are of the very closest kind. 



The ribs are situated in the septa between the great lateral 

 muscles of the body, and present much variation in the various 

 vertebrate Classes: they may be short and stump-like and almost 

 horizontal in position, or may grow ventralwards as delicate rod- 

 like structures, so as to encircle the body-cavity more or less 

 completely. Primitively, ribs may be present all along the 

 vertebral column, but, especially in the higher types, they become 

 reduced in certain regions. 



A careful study of the ribs, in which their relations to the soft 

 parts (muscles) is taken into consideration, shows that they are 

 not completely homologous throughout the vertebrate series, 

 and that those of most Fishes are not exactly morphologically 

 comparable to those of Elasmobranchii, Amphibia, and Amniota. 



Fishes. Two kinds of ribs, situated at different levels, may 

 be distinguished amongst Fishes dorsal ribs and ventral ribs (or 

 plcural arches} : the former extend into the transverse septa which 

 separate the epaxial or dorso-lateral from the hypaxial or ventro- 

 lateral muscles, while the latter are situated internally to the 

 muscles, just outside the peritoneum, but never more than 

 partially encircle the ccelome (Fig. 50). Both kinds of ribs are 

 usually considered as corresponding to prolongations of the transverse 

 processses (basal stumps) of the vertebral axis, from which they 

 have become segmented off but with which they remain closely 

 connected : another view as to their primary origin has been 

 stated above. The ventral ribs appear to be phylogenetically older 

 structures than the dorsal ribs, which can only have originated 

 after the differentiation of the intermuscular septa in which they 

 are situated. 



Towards the caudal region, the ventral ribs, together with the 

 corresponding transverse processes, gradually take on the form of 



