472 



Verteh-ata . 



last six cervical vertebrae) ^ and even liere they are only separate in 

 early life^ fusing later with the vertebra ; the posterior ones are not, 

 as in Eeptiles and Birds, longer than the anterior. In other Mammalia, 

 cervical ribs are, indeed, present, the so-called transverse processes of 

 the cervical vertebrae ; but they are at no time separated from the 

 vertebrae, so that their identity can only be recognised by comparison 

 with other forms. The thoracic vertebree are more sharply 

 marked oS from the cervicals than in Reptiles and Birds, since the 

 fii'st bears a movable rib, articulating with the sternum. In the 

 lumbar region there are usually fairly large transverse processes. 

 There are, as a rule, twenty thoracic and lumbar vertebrae (the 

 number may, however, sink to fourteen, or rise to thirty) ; the 

 thoracics are twelve or thirteen, but may rise to upwards of twenty. 

 Of true sacral vertebrae, i.e., those to which the ilia are 

 attached ; there are, as in Reptiles, usually only two fused together in 

 the adult, but in most Mammals one or more of the anterior caudal 

 vertebrae (the false sacral vertebrae) assist in forming the 

 sacrum. The caudal vertebrse vary considerably in number 



Pig. 383. Axis of a yoimg Platypus 

 (Ornithorhynchus) from tlie left side (A), 

 and from behind (B). 1 centrum of the 

 first cervical vertehra, 2 do. of the second, 

 b arch, r rib, t' inferior spine. In B the 

 arches, centrum, and ribs are shaded in. 

 different ways. — Orig. 



the anterior ones usually have well-developed transverse processes, dnd 

 often bear V-shaped bones, like those of many Reptiles, on the 

 ventral side ; the posterior tail vertebrae are always more or less 

 imperfectly developed, especially the last (arches and |)rocesses 

 degenerate). 



The ribs always consist of an upper and a lower portion, of which 

 the latter is usually cartilaginous or only partially ossified. In the 

 Monotremes, yet a third portion is intercalated (c/., the Crocodilia) 

 between these two. The majority of the ribs, the anterior so-called 

 true ribs, are attached to the sternum, whilst the posterior, or 

 so-called false ribs, are attached to one another and to the last 

 true rib, or terminate quite freely (floating ribs) . The rib articulates 

 with the transverse process of the corresponding thoracic vertebra by 

 an external outgrowth, the tuberculum (generally absent from the 

 posterior ribs) ; and with the centrum by the capitulum, the true 

 dorsal end of the rib. The articular facet lies upon one centrum, or 

 between that centrum and the one in front of it. The true ribs, of 

 which the first is usually especially strong, become longer towards the 



