THE VERTEBRA. 



283 



explanation of this rudimentary tail. In the human 

 embryo, indeed, during the earlier stages of germ-history, 

 the tail projects considerably. (Of. Plate VII. Fig. M n., 

 and Figs. 123, 8, 124, s, vol. i. p. 370.) It afterwards becomes 

 adherent, and is no longer externally visible. Yet traces 

 of the aborted tail- vertebrae, as well as of the rudimentary 

 muscles, which formerly moved them, persist throughout life. 

 According to the earlier anatomists the tail in the female 

 human being has one vertebra more than that of the male 

 (four in the latter, five in the former). 177 



The number of vertebrae in the human vertebral column 

 is usually thirty- three in all ; but it is an interesting fact 

 that this number frequently varies, one or another vertebra 

 failing, or a new, supernumerary vertebra inserting itself. 

 Not unfrequently, also, a rib, capable of free motion, forms 

 on the last neck-vertebra or on the first lumbar-vertebra, so 

 that thus there are thirteen breast, and six neck, or four 

 lumbar vertebrae. In this way contiguous vertebrae in the 

 different sections of the vertebral column may replace each 



