138 HUMAN EMBRYOLOGY AND MOEPHOLOGY. 



two muscles, the pubo-coccygeus and ischio-coccygeus (Fig. 109), 

 act as depressors of the tail, which in four-footed animals plays 

 the part of a perineal shutter. They are attached to the small 

 V-shaped chevron bones on the under surface of the basal caudal 

 vertebrae. Another muscle, the ischio- or spino-coccygeus, acts as 

 a lateral flexor of the tail (Fig. 109). It is attached to the trans- 

 verse processes of the caudal vertebrae, and rises from the dorsal 

 border of the ischium. In man the pubo-coccygeus and ilio- 

 coccygeus unite into one sheet and form the levator ani. The 

 shrinkage of the tail leaves the muscle partly stranded on the 

 ano-coccygeal ligament. Other fibres of the pubo-coccygeus loose 

 their primary insertion to the coccyx and become attached to the 

 prostrate, central point of the perineum and to the anal canal. 

 Both muscles, especially the ilio-coccygeus, retain in part their 

 primitive attachment to the coccyx (cauda). The spino-coccygeus, 

 or coccygeus muscle, is partly fibrous in man, its outer laminae 

 forming the small sacro-sciatic ligament ; its inner laminae remain 

 muscular and form the coccygeus. In man, too, the origin 

 of the ilio-coccygeus has sunk from the pelvic brim of the 

 ischium on to the obturator fascia (P. Thompson) ; traces of the 

 primitive origin from the pelvic brim can often be detected. The 

 white line, a structure peculiar to man, marks the new point of 

 origin of the levator ani from the obturator fascia. Further 

 details of changes undergone by the pelvic muscles and fasciae 

 may be found in papers by Dr. P. Thompson in the Journal of 

 Anatomy and Physiology, vol. xxxv. 



On the dorsal and ventral aspects of the sacrum and coccyx, 

 fibrous or muscular vestiges of the anterior and posterior sacro- 

 coccygeal muscles (elevators and flexors of the tail) are commonly 

 to be found in man. 



The Pelvic Fascia and Fasciae in General. — It has been cus- 

 tomary to regard fasciae as separate structures forming distinct 

 sheets with devious and complex courses. It is possible by 

 dissection to prepare and display them according to accepted 

 descriptions, but the structures so displayed are artificial and 

 not the true structures with which the surgeon or physician 

 has to deal with in actual practice. Embryology is the best 

 guide to their nature. Take the development of the fasciae seen 

 on making a section of the upper arm, for example. When the 



