Lesson 11 - Pelvis and Perineum
As I noted in lesson #10, the abdomen and pelvis are continuous with one another, and many of the abdominal organs actually rest in the pelvis when a person is standing upright. Because of this, the abdomen and pelvis together are sometimes referred to as the abdominopelvic cavity. In this lesson, I will review the anatomy of the bony pelvis, and cover muscles that span the pelvic inlet and outlet and provide passage for structures, like the rectum, that make up the distal end of the GI tract, in addition to the end of the urinary tract and the reproductive organs.
Learning Objectives - By the end of this lesson, you should be able to ...
- Describe the anatomy of the bony pelvis
- Describe how to distinguish a male pelvis from a female pelvis
- List and describe the ligaments and apertures of the pelvis
- Understand the organization of layers in the pelvic floor and the borders of the perineum
- Describe the origins, insertions, innervation, and function of muscles in the pelvic floor and perineum
The pelvic girdle is a ring made up of three bones: two hip, or coxal, bones, which are sometimes also called ossa coxae, innominates, or simply "pelvic" bones; and the sacrum, which is part of the vertebral column. The term pelvis can be used loosely to refer to this collection of three bones, or it can just refer to the innominate/ossa coxae part. The pelvis is a complex 3-D bowl-shaped structure that has muscles and ligaments slung from its internal borders which separate the pelvic inlet from the abdominal cavity and the pelvic outlet from the perineum. Each innominate is made up of three bones: the ilium, ischium, and pubis, that are joined together in the adult at the acetabulum, or hip joint (this is where the head of the femur fits into the innominate). Viewed laterally, the other obvious features of an innominate include the iliac crest, where the gluteal muscles attach, and the obturator foramen, a large hole through which various vessels pass to get to the thigh.
- Ilium: this is the largest of the three bones. Externally and laterally, the iliac crest and gluteal surface of the ilium provides attachment for the hip extensors and AB-ductors (i.e., the gluteal muscles). Internally, the iliac fossae participate (along with the sacrum) to make the bowl-shaped structure within which the small intestines and parts of the large intestines sit. A few important features of the pelvis include the anterior and posterior superior iliac spines and the anterior and posterior inferior iliac spines, which provide attachment areas for numerous muscles and ligaments. The greater sciatic notch is just inferior to the posterior inferior iliac spine, and is very useful for distinguishing male from female pelves. Each ilium is connected to the sacrum at the sacroiliac joint, which does not permit very much movement (except slight rotation of the sacrum and coccyx relative to the ilia during childbirth).
- Ischium: The ischium is the inferoposterior portion of the hip - the part on which you sit! Posteriorly, the ischium is heavily-built, and the ischial tuberosity is the part of the ischium on which you actually sit. A ramus (Latin for "bar") called the ischiopubic ramus connects the ischium and pubis. Other important features of the ischium include the lesser sciatic notch and ischial spine.
- Pubis: the pubis is the most anterior portion of the pelvis, and is made up of a body and superior and inferior rami (the plural of "ramus"). The two pubic bones fit tightly together at the pubic symphysis, a symphyseal (fibrocartilaginous) joint. This symphysis becomes "looser" during childbirth.
Sexing the Pelvis
The pelvis is the best bone in the body with which to determine the sex of an individual from the skeleton, since the female pelvis is adapted to passing a large-headed infant during childbirth. Below is a list of features commonly used to sex the pelvis, but students should understand that this is an area of active investigation in forensic anthropology and human osteology, so there are many more characteristics than just the ones listed here. When perusing the list below, keep in mind that most of the "female" characteristics are related to a wider pelvis, sacrum, and pelvic inlet and outlet.
- Size: large and rugged in males, generally small and gracile in females
- Ilium: high and vertical iliac blades in males, low, flat, and wide in females
- Pelvic inlet: generally "heart-shaped" in males, circular or elliptical in females
- Subpubic angle: "V"-shaped in males, wider and "U"-shaped in females
- Obturator foramen: large and ovoid in males, small and triangular in females
- Preauricular sulcus: rare in males, generally present and well-developed in females.
- Shape of sacrum: long and narrow in males, short and broad in females
- Greater sciatic notch: narrow in males, wide in females
Pelvic Ligaments and Apertures
Several ligaments make up parts of the pelvic wall, provide attachment areas for muscles, and form foramina through which nerves, arteries, veins, and other structures run. Here we will cover the major ligaments and spaces.
- Sacrospinous ligament: runs from the ischial spine to the margins of the sacrum and coccyx.
- Sacrotuberous ligament: runs from the posterior superior iliac spine, lateral margin of the sacrum, and lateral surface of the coccyx to the medial margin of the ischial tuberosity. This ligament is in a superficial position relative to the sacrospinous ligament, and the two ligaments are at approximate right angles to one another.
- Greater sciatic foramen: the walls of the greater sciatic foramen are made up of the greater sciatic notch between the iliac blade and acetabulum anteriorly, the sacrospinous ligament and ischial spine inferiorly, and the sacrotuberous ligament posteriorly. A small muscle called the piriformis muscle passes through the greater sciatic foramen and splits it into two parts: above the piriformis, the superior gluteal nerves and vessels pass from the pelvic cavity out to the gluteal muscles, which line the "outer" gluteal surface of the ilium. Below the piriformis, the inferior gluteal nerves and vessels, the sciatic nerve, pudendal nerve, internal pudendal vessels, posterior femoral cutaneous nerves, and nerves to the obturator internus and quadratus femoris muscles.
- Lesser sciatic foramen: the boundaries of this foramen are formed by the lesser sciatic notch of the ischium anteriorly, the sacrospinous ligament superiorly, and the sacrotuberous ligament posteriorly. The obturator internus tendon passes from the pelvic cavity to the gluteal region through the lesser sciatic foramen. The pudendal nerve and internal pudendal vessels run out of the pelvic cavity through the greater sciatic foramen, loop around the ischial spine and sacrospinous ligament, and enter the perineum through the lesser sciatic foramen.
- Obturator canal: this canal is bordered by the obturator membrane, obturator internus and obturator externus muscles, and the superior pubic ramus. The obturator nerve and vessels pass from the pelvic cavity to the thigh through the obturator canal.
Muscles of the Pelvic Floor
There are three layers (or compartments) of muscles and fascia in the pelvic floor. It is important to realize that the anal canal, urethra and, in females, the vagina, pierce all three layers. The perineum is the diamond-shaped region between the thighs that forms the borders for the muscles in the pelvic floor. The perineum is bordered anteriorly by the pubic symphysis, posteriorly by the coccyx, and laterally by the ischial tuberosities. The anterior half of the perineum is known as the urogenital triangle, since it passes the distal parts of the urinary tract and reproductive system, and the posterior half is known as the anal triangle since it passes the anal canal.
Superficial Perineal Space - 3 muscles
This is the most superficial of the three muscle layers of the pelvic floor. The superficial perineal space contains the ischiocavernosus, bulbospongiosus, and superficial transverse perineus muscles.
- Ischiocavernosus: covers the crura of the penis and clitoris. This muscle, which originates on the ischial tuberosity and ramus and inserts onto the crus of the penis or clitoris, is innervated by S2 to S4 fibers of the pudendal nerve. It moves blood to erect the penis or clitoris.
- Bulbospongiousus: originates on the perineal body and inserts onto the corpus cavernosum and surrounding structures of the penis or clitoris. It is innervated by S2 to S4 fibers of the pudendal nerve. It moves blood to the penis or clitoris and is active during ejaculation.
- Superficial transverse perineus: originates on the ischial tuberosity and ramus and inserts onto the perineal body. LIke the ischiocavernosus and bulbospongiosus muscles, it is innervated by S2 to S4 fibers in the pudendal nerve. It functions to stabilize the perineal body.
Middle Compartment - 3-4 muscles
The deep transverse perineus, external urethral sphincter, and anal sphincter lie in this compartment. The compressor urethrae muscle also resides in this compartment in females. Note that Gray's Anatomy for Students 3e and Gray's Basic Anatomy 2e call this compartment the deep perineal pouch.
- Deep transverse perineus: lies deep to the superficial transverse perineus muscle, running from the ischial ramus to the perineal body in the middle compartment. Like other perineal structures, it is innervated by S2 to S4 fibers of the pudendal nerve. Like the overlying superficial transverse perineus muscle, it stabilizes the position of the perineal body.
- External urethral sphincter: a circular muscle which surrounds the membranous part of the urethra and compresses it; relaxes during micturation. It runs from the inferior ramus of the pubis on each side to surround the membranous urethra. It is innervated by S2 to S4 branches of the pudendal nerve.
- External anal sphincter: a circular muscle which surrounds the anal canal and compresses it; relaxes during defecation.
- Compressor urethrae: runs from the ischiopubic ramus on each side to blend with its partner anterior to the urethra. Like other perineal structures, it is innervated by S2 to S4 branches of the pudendal nerve. It functions in an accessory role to close the urethra.
Pelvic Diaphragm - 2 groups
The pelvic diaphragm is made up of the levator ani muscle group (which includes the puborectalis, pubococcygeus, and iliococcygeus muscles), and the coccygeus muscle. The levator ani muscles are all innervated directly by branches from anterior rami of the S4 spinal nerve and S2 to S4 branches of the pudendal nerve. These muscles contribute to the pelvic floor.
- Puborectalis: runs from the pubic bone to form a sling around the inferior end of the GI tract at the junction between the rectum and anal canal.
- Pubococcygeus: runs from the pubic bone to the coccyx, surrounding midline structures just lateral to puborectalis. Various parts of this muscle surround the prostate or vagina and the anal canal.
- Iliococcygeus: runs from the fascia covering the obturator internus muscle to meet its partner in the midline and form a raphe that extends from the anal canal to the coccyx.
- Coccygeus: runs from the ischial spine and pelvic surface of the sacrospinous ligament to the lateral margin of the coccyx and contiguous border of the sacrum. It is innervated by S3 and S4 anterior rami of spinal nerves, and pulls the coccyx forward after defecation.
In the next lesson, we cover pelvic organs, and then we move on to the head.
- Human Anatomy Lesson 12
This lesson covers anatomy of the organs in the pelvic cavity, including the rectum, anal canal, bladder, ureters, urethra, and components of the male and female reproductive systems.
- Human Anatomy Lesson 13
This lesson covers the superficial tissues of the head, including the muscles of facial expression and the nerves, arteries, veins, and glands.
- Human Anatomy Lesson 14
This lesson covers the skull, which is the bone that surrounds the sensory organs in the head.
- Human Anatomy Lesson 15
This lesson covers the 12 cranial nerves, including their origin, route of passage through the skull base, and the things that they innervate.
- Human Anatomy Lesson 16
This lesson covers the brain and its meninges and vasculature.
- Human Anatomy Lesson 17
This lesson covers the temperomandibular joint and the structures in the temporal and infratemporal fossae.
- Human Anatomy Lesson 18
This lesson covers the anatomy of the orbit, eye, and ear. It is the 18th lesson in a comprehensive undergraduate Human Anatomy course.
© 2014 Robert McCarthy