138 HISTORY OF THE HUMAN BODY 



losed to the vertebrae which bear them, leaving a set of 

 thoracic vertebrae (the " dorsal " vertebrae of the older termin- 

 ology) interpolated between two groups, cervical and lumbar, 

 in which the rib elements are fused. In the cervical vertebrae 

 these fused ribs form the ventral element of the plainly double 

 transverse processes, and enclose between themselves and the 

 original transverse process (diapophysis) the vertebral for- 

 amina. In the lumbar vertebrae the rib elements form the 

 large wing-like transverse processes (pleurapophyses), and 

 are thus seen to be not equivalent to the processes of the same 

 name in other regions. 



The number, both of free ribs and of vertebrae forming 

 each group, differs considerably, not only in different mam- 

 mals, but even in different individuals of the same species. 

 Thus in Man, although twelve pairs of free ribs is the rule, 

 "the rib element of the last (7th) cervical vertebra is occasion- 

 ally free, " cervical rib!' and, more commonly, a free rib ap- 

 pears on the first lumbar vertebra. As this is perhaps the 

 rule rather than the exception in the gorilla, one of Man's 

 nearest living allies, this anomaly is often called the "gorilla 

 rib." Variation in the sacral vertebrae has already been 

 noticed [Cf. Fig. 34 and accompanying text]. 



The origin of the sternum is still a matter of controversy, 

 and it seems likely that there may have been two sternums, 

 of different origin, the one succeeding the other during his- 

 torical development, the archisternum and the neosternum. 

 Fishes lack the part entirely, but it is present in some form 

 in all other vertebrates save in a few aberrant cases, for ex- 

 ample, the snakes, where it is incompatible both with their 

 mode of locomotion by means of the ends of the very numer- 

 ous ribs, and with their habit of swallowing huge mouth f tils, 

 far too large to pass through the ring formed by the vertebrae, 

 ribs, and sternum, as is the usual arrangement. What is ap- 

 parently the first indication of a sternum is seen in the sala- 

 mander Necturus, perhaps the lowest amphibian, in which 

 from three to five of the thoracic myocommata chondrify in 

 the ventral region, forming small V-shaped elements, indefi- 



