Chap. I.] RUDIMENTS. 29 



external rudiment of a tail. The os coccyx is short, usu- 

 ally including only four vertebrae ; and these are in a rudi- 

 mental condition, for they consist, with the exception of 

 the basal one, of the centrum alone. 40 They are furnished 

 with some small muscles ; one of which, as I am informed 

 by Prof. Turner, has been expressly described by Theile as 

 a rudimentary repetition of the extensor of the tail, which 

 is so largely developed in many mammals. 



The spinal cord in man extends only as far downward 

 as the last dorsal or first lumbar vertebra ; but a thread- 

 like structure (the filum terminale) runs down the axis of 

 the sacral part of the spinal canal, and even along the back 

 of the coccygeal bones. The upper part of this filament, 

 as Prof, Turner informs me, is undoubtedly homologous 

 with the spinal cord ; but the lower part apparently con- 

 sists merely of the pia mater, or vascular investing mem- 

 brane. Even in this case- the os coccyx may be said to 

 possess a vestige of so important a structure as the spinal 

 cord, though no longer enclosed within a bony canal. The 

 following fact, for which I am also indebted to Prof. 

 Turner, shows how closely the os coccyx corresponds with 

 the true tail in the lower animals : Luschka. has recently 

 discovered at the extremity of the coccygeal bones a very 

 peculiar convoluted body, which is continuous with the 

 middle sacral artery ; and this discovery led Krause and 

 Meyer to examine the tail of a monkey (Macacus) and of 

 a cat, in both of which they found, though not at the ex- 

 tremity, a similarly convoluted body. 



The reproductive system offers various rudimentary 

 structures ; but these differ in one important respect from 

 the foregoing: cases. We are not here concerned with a 

 vestige of a part which does not belong to the species in 

 an efficient state ; but with a part which is always present 

 and efficient in the on.e sex, being represented in the other 



*° Owen, 'On the Nature of Limbs,' 1849, p. 114 



