562 



HUMAN ANATOMY. 



Variations.— The pyriformis is occasionally absent, and it may be more or less fused with 

 the gluteus minimus or medius. Frequently it is divided into two or more portions by being 

 oerforated by the sciatic nerve. 



From the comparative stand-point the pyriformis is to be regarded, in part at all events, as 

 a portion of the musculature extending between the axial skeleton and the pelvic girdle or limb, 

 and is represented in the lower vertebrates by the caudo-femoralis. 



{b) THE PERINEAL MUSCLES. 



I. 



2. 



3- 



Sphincter ani externus. 

 Transversus perinaei superficialis. 

 Ischio-cavernosus. 



4. Bulbo-cavernosus. 



5. Transversus perinaei profundus. 



6. Compressor urethrae. 



In the early stages of development, while the urogenital ducts and the digest- 

 ive tract open into a common terminal cavity, the cloaca, muscle-fibres derived 

 from the second, third, and fourth sacral myotomes arrange themselves in a flat 

 ■layer around the external aperture of the cavity, forming what is termed the 

 sphincter cloactz. Later, with the division of the cloaca into a urogenital and a 

 rectal portion and the resulting formation of the perineum, this primary sphincter 

 becomes divided into two portions, one of which forms a sphincter ani, while the 

 more anterior portion gives rise to the muscles of the perineum. The fibres of this 

 latter portion undergo various modifications in accordance with the changes which 



Fig. 553. 



.Peritoneum 



Urachus. 

 Supravesical space 



Symphysis pubis 



Suspensory ligament 

 Triangular ligament, sup 

 Deep perineal interspa 

 Triangular ligam't, inf. lay 



Urethra 

 Penis, corpus cavernosum 



Scrot 



Bladder I 



;_ Rectum 



Prostate 



Cowper's gland 



Perineal centre 

 Giles's fascia 

 Superficial perineal interspace 



Colles's fascia 



Diagrammatic sagittal section, showing relations of fascial layers of perineum. 



take place in the urogenital sinus, and a horizontal separation of the original 

 sphincter into two layers also occurs, whereby the perineal muscles are arranged in 

 two layers separated by the superior fascia of the urogenital trigone. 



The muscles formed during these changes retain the original sheet-like form of 

 the sphincter cloacae and are for the most part pale in color, resembling not a little in 

 their general character the platysma muscles of the face. They show a considerable 

 amount of difference in their development in different individuals, numerous acces- 

 sory muscles having been described by various authors, some of which will be 

 referred to in the succeeding descriptions. 



The Superficial Perineal Fascia. — The superficial perineal fascia, being 

 continuous anteriorly with the superficial fascia of the lower portion of the anterior 

 abdominal wall, is, like this, composed of two layers. The more superficial layer 

 usually contains a certain amount of fat, and, as in the abdomen, is really the pan- 

 niculus adiposus of the skin. The deeper layer, which has been termed the fascia 

 of Colics, forms a continuous membrane which is attached at the sides to the rami 

 of the pubes and ischia and in front becomes continuous with the dartos of the 



