SlOLOGICAL BEGINNINGS 2^ 



ening may, I presume, be attributed to civilised men habitually feeding on 

 soft, cooked food, and thus using their jaws less. 



With respect to the alimentary canal, I have met with an account of only 

 a single rudiment, namely the vermiform appendage of the caecum. The 

 caecum is a branch or diverticulum of the intestine, ending in a cul-de-sac, 

 and is extremely long in many of the lower vegetable-feeding mammals. 

 In the marsupial koala it is actually more than thrice as long as the whole 

 body. It is sometimes produced into a long gradually-tapering point, and 

 is sometimes constricted in parts. It appears as if, in consequence of changed 

 diet or habits, the caecum had become much shortened in various animals, 

 the vermiform appendage being left as a rudiment of the shortened part. 

 That this appendage is a rudiment, we may infer from its small size, and 

 from the evidence which Prof. Canestrini has collected of its variability 

 in man. It is occasionally quite absent, or again is largely developed. The 

 passage is sometimes completely closed for half or two-thirds of its length, 

 with the terminal part consisting of a flattened solid expansion. In the orang 

 this appendage is long and convoluted: in man it arises from the end of 

 the short caecum, and is commonly from four to five inches in length, 

 being only about the third of an inch in diameter. Not only is it useless, 

 but it is sometimes the cause of death, of which fact I have lately heard two 

 instances: this is due to small hard bodies, such as seeds, entering the pas- 

 sage, and causing inflammation. 



In man, the os coccyx, together with certain other vertebrae hereafter to 

 be described, though functionless as a tail, plainly represent this part in 

 other vertebrate animals. At an early embryonic period it is free, and pro- 

 jects beyond the lower extremities 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, Vv'ith the exception of the basal one, of the 

 centrum alone. 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, a muscle which is 

 so largely developed in many mammals. 



The spinal cord in man extends only as far downwards as the last dorsal 

 or first lumbar vertebra; but a thread-Hke 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 consists merely of the pia mater, or vascular in- 

 vesting membrane. 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. 



