146 MAMMALIAN ANATOMY 



The ribs are subject to marked variations in the development of 

 their peculiar features. In some specimens the surfaces are deeply 

 impressed by muscular attachments, and the prominent borders de- 

 velop accessory angles ; in other specimens a few leading features only 

 are distinguishable, all others have disappeared. 



The eleventh rib frequently exhibits a tubercle, more or less 

 marked, which articulates with a rudimentary vertebral transverse 

 process (Figs. 54, 55). This variation is independent of variations 

 in the number of the ribs. 



In one specimen of very many examined, the vertebral part of the 

 caudal-medial surface of the tenth rib is deeply grooved for the inter- 

 costal vessels ; an interesting variation, because it reproduces a normal 

 feature of the human ribs. 



In another specimen the heads of the twelfth and thirteenth ribs 

 of both sides have a distinct vertebral surface deeply grooved trans- 

 versely to the long axis of the rib. Inasmuch as the articular facets 

 on the corresponding vertebrae exhibit no peculiarities, the groove proba- 

 bly transmitted tendons or vessels. 



VARIATIONS IN NUMBER. 



The number of ribs may be increased to fourteen by the addition 

 of a pair attached to the first lumbar vertebra. In the one example 

 of this variation which I have preserved (No. 5 in the table), the 

 additional ribs resemble enlarged and somewhat modified lumbar trans- 

 verse processes rather than true ribs ; they are not round, but flattened, 

 especially at the vertebral end, which is four times as wide as the free 

 end ; they are not bowed, but exhibit merely a faint sinuous cephalo- 

 caudal curve. 



The occurrence of cervical ribs attached to the seventh cervical 

 vertebra is not very rare in man, but so far as known they do not 

 occur in the cat. This is not surprising, because the seventh cervical 

 vertebra normally lacks the arterial canal and the costal element in its 

 transverse process, features which we should expect to find permanently 

 developed before the cervical ribs appear. 



So far as known, the number of ribs in the cat is never less than 

 thirteen pairs ; the thirteenth pair, however, are sometimes so greatly 

 reduced in size that their entire absence would not be remarkable. 



