138 COMPARATIVE ANATOMY. 



Eibs. 

 § 335. 



The name of ribs is given to those parts of the skeleton which 

 have been developed from the inferior arches of the vertebra?, and 

 which are either permanently, or transitorily, articulated to the 

 vertebral column. As a rule they clasp round and enclose a sub- 

 vertebral cavity. This cavity is divisible into two portions, which 

 differ in size, and in the organs which they contain. The anterior 

 one is the coelont. The posterior one is continued into the tail, and 

 forms the narrow caudal canal, which is sometimes divided hori- 

 zontally into two parts. These relations may be seen in Fishes, where 

 also the division of the body into regions is in its most indifferent 

 condition. 



A comparison of the contents of the two tracts of the sub- 

 vertebral cavity will explain their difference in size. While in the 

 caudal canal there is nothing but blood-vessels, or, at most, parts of 

 the kidneys, which are always organs that vary but little in size, 

 great variations in volume can be made out in the organs of the 

 coelom, and these variations may be often seen to be due to a regular 

 succession of states, in which the organs are being filled or emptied. 

 The arrangements which can be made out in the lower arches are 

 correlated with this characteristic. They arc seen to be direct 

 processes of the vertebra in the caudal region, and are immovable. 

 In the abdominal region, however, in adaptation to the varying 

 size of the cavity they enclose, they are segmented off from the 

 vertebrae, and are more or less movably articulated to the centra or 

 its processes (transverse processes). 



The ribs are therefore regarded as differentiations of 

 the inferior system of arches; a number of arches, varying 

 according to the extent of the coelom, have been converted into the 

 more free form of rib. This view, which explains the origin of the 

 ribs, proves to us that the inferior arches, which resemble the 

 ribs, but which do not now enclose the coelom, are not primitive 

 structures, but that they once were ribs; this necessarily pre- 

 supposes that the coelom once extended much farther back. 



§ 330. 



As we have already treated of the indifferent lower arches I 

 when speaking of the vertebral column, we have now only to deal 

 with the ribs and the organs derived from them. They are not 

 completely absent in any of the Vertebrata, save the Leptocardii, 

 Cyclostomata^ and the Chimasra?. In all the rest they are either in a 

 rudimentary or in a well-developed condition; in the latter case they' 

 may become connected together on the ventral surface, and a special 

 skeletal piece, the sternum, is then developed. 



