THE SPINOUS PROCESSES 



103 



have already seen that in the Crocodile (and in some other 

 existing Reptiles) another upright spine is beginning to 

 be evident at the hind end of the rib series, and this is 

 the one to which the term anticlinal vertebra is usual ly 

 applied in mammalian anatomy. 



Within the limits of the Mammalia, the condition of 

 the trend of the spinous processes varies enormously. 

 Among the Prototheria, Echidna and Proechidna show a 

 series of cervical, dorsal, and lumbar spines which point 

 uniformly backwards towards the sacral region (see 

 Fig. 35). This is apparently the primitive mammalian 



*4i*A4i' 



Fig. 35. — Diagram of the Vertebral Column of an Animal 

 IN WHICH All the Spinous Processes are Retroverted. 



as well as the primitive reptilian condition, and, as such, 

 is seen in an extraordinarily varied collection of extinct 

 species embracing such forms as Toxodon, Arsinotherium, 

 Mylodon, etc. In Ornithordhynchus, there is a change, 

 for though all the cervical and dorsal spines slope acutely 

 backwards, those of the three lumbar vertebras slope 

 forwards, there being an anticlinal element at the hind 

 end of the rib-bearing series. In the Metatheria and 

 Eutheria, the very widest divergence in spinal inclination 

 is seen, and it seems most probable that some functional 

 demand determines the variations met with. Owen 

 paid attention to this point, and recognized clearly the 



