214 HISTORY OF THE HUMAN BODY 



In the tailless apes and in man, corresponding to the great 

 reduction of caudal (coccygeal) vertebrae, the muscles are 

 greatly reduced, and of uncertain occurrence, yet traces of all 

 except the inter transversarii have been detected, and certain of 

 them are fairly constant. The sacro-coccygei posteriores (ex- 

 tensores coccygis) are found upon the dorsal side of sacrum 

 and coccyx, and in individual cases may represent either of the 

 two extensors, or the dorsal abductor, or any combination of 

 these. Thus in 100 human bodies the medial extensor oc- 

 curred 53 times, the lateral extensor 43 times and the dorsal 

 abductor 87 times, there being but six cases in which indica- 

 tions of the group are wholly absent. Upon the ventral side 

 of the coccyx are found rudiments of the ftexores caudcz, de- 

 scribed under the names of sacro-coccygei anterior es, s. curva- 

 tores coccygis, and of the ventral abductor, here called the 

 coccygeus or abductor coccygis. The former was found in 

 1 02 out of 1 10 cases, the latter appears to be constant, although 

 often tendinous. 



Two further caudal muscles, pubo-coccygeus and ilio-coccy- 

 geus, are found in long-tailed mammals, stretching partly 

 across the pelvic floor, and sustaining some connection with 

 the rectum at its termination. These muscles, although rather 

 small, possess much morphological interest, since, in the an- 

 thropoid apes and in man, they come to lie transversely across 

 the posterior pelvic opening and form the levator ani, a muscle, 

 which, in connection with the coccygeus, is the principal ele- 

 ment concerned in the construction of the so-called diaphragma 

 pelvis. This is a muscular and tendinous floor or partition, 

 closing the posterior outlet of the pelvic cavity, and its forma- 

 tion is unquestionably an adaptation to the erect position of 

 the anthropoids and man, thus strengthening what would 

 otherwise become from this position a point of weakness. 



Although the exact homologies of pubo- and ilio-coccygeus 

 (levator ani) are still somewhat obscure, they probably belong 

 to the ventral axial system, and are not connected otherwise 

 than by contiguity with the other muscles of the perinseum, 

 and the genital organs, such as the sphincter ani, ischio-cav- 



