RUDIMENTS. 25 



nearer in the long line of descent to their remote animal- 

 like progenitors. 



In man, the os coccyx, together with certain other verte- 

 brae hereafter to be described, though f unctionless as a tail, 

 plainly represent this part in other vertebrate animals. At 

 an early embryonic period it is free, and projects beyond 

 the lower extremities; as may be seen in the drawing (Fig. 

 1) of a human embryo. Even after birth it has been known, 

 in certain rare and anomalous cases,* to form a small ex- 

 ternal rudiment of a tail. The os coccyx is short, usually 

 including only four vertebrae, all anchylosed together; and 

 these are in a rudimentary condition, for they consist, with 

 the exception of the basal one, of the centrum alone, f 

 They are furnished with some small muscles; one of which, 

 as I am informed by Prof. Turner, has been expressly de- 

 scribed by Theile as a rudimentary repetition of the exten- 

 sor of the tail, a muscle 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 fihim 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 consists merely 

 of the j)i(^ mater, or vascular investing membrane. Even 

 in this case the os coccyx may be said to possess a ves- 

 tige of so important a structure as the spinal cord, though 

 no longer inclosed 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 dis- 

 covered at the extremity of the coccygeal bones a very pe- 

 culiar convoluted body, which is continuous with the mid- 

 dle sacral artery; and this discovery led Krause and Meyer 



*Quatrefages has lately collected the evidence on this subject. 

 " Revue des Cours Scientifiques," 1867-1868, p. 625. In 1840 Fleisch- 

 luann exhibited a human foetus bearing a free tail, which, as is not 

 always the case, included vertebral bodies; and this tail was critically 

 examined by the many anatomists present at the meeting of natural- 

 ists at Erlangen (see Marshall in ' ' Niederlandischen Archiv fiir Zoolo- 

 gie," December, 1871). 



f Owen, " On the Nature of Limbs," 1849, p. 114. 



