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Henry Gray (1825–1861).  Anatomy of the Human Body.  1918.
 
the Obturator internus and are enclosed in a special canal—Alcock’s canal—formed by the obturator fascia.
  The fascia of the Piriformis is very thin and is attached to the front of the sacrum and the sides of the greater sciatic foramen; it is prolonged on the muscle into the gluteal region. At its sacral attachment around the margins of the anterior sacral foramina it comes into intimate association with and ensheathes the nerves emerging from these foramina. Hence the sacral nerves are frequently described as lying behind the fascia. The internal iliac vessels and their branches, on the other hand, lie in the subperitoneal tissue in front of the fascia, and the branches to the gluteal region emerge in special sheaths of this tissue, above and below the Piriformis muscle.


FIG. 402– Coronal section of pelvis, showing arrangement of fasciæ. Viewed from behind. (Diagrammatic.) (See enlarged image)

  The diaphragmatic part of the pelvic fascia (Fig. 402) covers both surfaces of the Levatores ani. The inferior layer is known as the anal fascia; it is attached above to the obturator fascia along the line of origin of the Levator ani, while below it is continuous with the superior fascia of the urogenital diaphragm, and with the fascia on the Sphincter ani internus. The layer covering the upper surface of the pelvic diaphragm follows, above, the line of origin of the Levator ani and is therefore somewhat variable. In front it is attached to the back of the symphysis pubis about 2 cm. above its lower border. It can then be traced laterally across the back of the superior ramus of the pubis for a distance of about 1.25 cm., when it reaches the obturator fascia. It is attached to this fascia along a line which pursues a somewhat irregular course to the spine of the ischium. The irregularity of this line is due to the fact that the origin of the Levator ani, which in lower forms is from the pelvic brim, is in man lower down, on the obturator fascia. Tendinous fibers of origin of the muscle are therefore often found extending up toward, and in some cases reaching, the pelvic brim, and on these the fascia is carried.
  It will be evident that the fascia covering that part of the Obturator internus which lies above the origin of the Levator ani is a composite fascia and includes the following: (a) the obturator fascia; (b) the fascia of the Levator ani; (c) degenerated fibers of origin of the Levator ani.

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