136 HUMAN EMBEYOLOGY AND MOEPHOLOGY. 



(1) In the second month the coccygeal region of the spine 

 protrudes (Fig. 108). 



(2) Vestiges of the extensor and flexor muscles of the tail are 

 frequently found (10°/ o of bodies) on the dorsal and ventral 

 aspects of the sacrum and coccyx. 



(3) True tails, consisting of external prolongations of the 

 coccyx, commonly fibrous, rarely containing vertebrae, occasionally 

 occur. 



Wolf. duct. 



rect jm TMI. • . • ; • ; ':- '■'••A — cor d- 



! ~allantois 



cloacal membrane 

 post. anal, gut 

 — cauda 



Fig. 108; — The caudal end of the body in a human embryo of the 3rd week. 



(4) The post-anal pit, always to be seen in the newly-born 

 child, marks the point at which the coccyx disappears below the 

 surface. In man the coccyx forms part of the perineal floor. 

 Instead of projecting far beyond the gut, as in tailed mammals, 

 it terminates 1\ inches above the commencement of the anal 

 canal. 



The Pelvic Floor is peculiarly extensive in man, an adaptation 

 to his upright posture. The floor is formed by the following 

 structures : 



(1) The levator ani and its sheath (recto- vesical and anal 

 fasciae) on each side. 



(2) The coccyx and coccygeus muscles. 



(3) The constrictor urethrae and its sheath (the triangular 

 ligament). 



(4) The pyriformis and its sheath may also be included. 

 Development of the Pelvic Floor.— The pelvic floor has been 



evolved in man by a transformation of the tail and the caudal 



