of the muscle it is continuous with the general fascial lining of the pelvic cavity, and gives rise to a conspicuous thickening, the tendinous arch (arcus tendineus) of the pelvic fascia, which stretches like a bow-string from the back of the symphysis pubis to the ischial spine. This band is related not so much to the origin of the levator ani muscle, which often extends higher up external to the pelvic fascia, as to the attachments of the fascial investments of the genito-urinary passages, to be described below. There are sometimes additional thickenings of the fascia, branching upwards from the tendinous arch towards the superior aperture of the pelvis. At the insertion of the levator ani, the fascia clothing its pelvic surface is attached to the perineal body, the margin of the anal canal, and the ano-coccygeal body, over which it passes to be continuous, above the raphe of insertion of the levatores ani, with the layer of the opposite side. At the antero

FIG. 435.-RELATIONS OF PELVIC FASCIA TO THE RECTUM AND PROSTATE
(Median Section of the Pelvis).

inferior border of the muscle the fascia enclosing it become continuous with the superior fascia of the urogenital diaphragm; at its postero-superior border they join the fascia enclosing the coccygeus muscle.

Within the pelvic basin, the walls and floor of which are thus continuously invested by the pelvic fascia, are contained the rectum and bladder, and in the female the uterus, suspended and maintained in position by the peritoneum, extra-peritoneal tissue, and the pelvic vessels and nerves. They are essentially free to distend or collapse, and are not bound down by the pelvic fascia. The rectum in both sexes extends down to the floor of the pelvis, where the anal canal takes its origin. It is invested by the peritoneum and extra-peritoneal tissue, and occupies a special rectal channel; this is lined by pelvic fascia, which gains an attachment to the floor of the pelvis at the margin of the anal canal.

The arrangement of the fascia in relation to the genito-urinary passages is essentially different.

Just as from the perineal aspect the inferior aperture of the pelvis is divisible into two different parts,-a posterior or dorsal part, comprising the ischio-rectal

fossæ for the passage of the anal canal, and characterised by looseness and distensibility; and an anterior or ventral part, the urethral triangle for the genitourinary passages, and characterised by firm fixation to the pubic bones; so also from the abdominal aspect it is found that, while in the posterior part of the pelvis a rectal channel exists, in which the rectum is free to collapse and distend, in the ventral part of the basin the genito-urinary passages are firmly fixed by means of

FIG. 436.-RELATIONS OF THE PELVIC FASCIA TO THE RECTUM, URETHRA, AND VAGINA (Median Section).

the pelvic fascia, which gives rise to a special suspensory ligament for the prostate gland and the prostatic urethra in the male, and for the urethra and vagina in the female.

A crescentic fold of pelvic fascia (suspensory ligament) arises in the neighbourhood of the sciatic spine from the general fascia covering the pelvic wall. It has a posterior free edge, through which the ductus deferens, vesical vessels, and nerves pass. Sweeping across the median plane, this border is continuous with the fold of the opposite side, the two together constituting the anterior limit of the rectal

channel. The fascial fold is composed of two layers, posterior and superior, between which is a large plexus of veins. They have separate attachments laterally to the general pelvic fascia. The posterior (recto-vesical) layer passes across the pelvis between the prostate gland and the rectum. Its inferior edge is attached to the perineal body between the base of the fascia of the urogenital diaphragm and the beginning of the anal canal. It forms a sheath for the vesiculæ seminales and ductus deferentes. This is rather in the form of a septum than a complete sheath; it effectually separates the vesiculæ seminales and the bladder from the rectum, forming the anterior wall of the rectal channel, but it allows the vesiculæ seminales to rest directly against the bladder. The superior layer extends forwards to the symphysis pubis. It has a lateral origin from the arcus tendineus in its whole length, and sweeping over the prostate gland, it is inserted along its line of junction with the bladder, and constitutes the so-called lig. puboprostaticum laterale (lateral true ligament of the bladder). It contains numerous bundles of muscular fibres in its anterior part, and forms a sheath for the passage of the inferior vesical vein along the lateral surface of the prostate gland. In front the fascia stretches from the back of the symphysis pubis, the arcuate ligament of the pelvis, and the superior fascia of the urogenital diaphragm to the neck of the bladder and the prostate gland, forming the lig. puboprostaticum medium. It is continuous across the median plane with the ligament of the opposite side. In the median line, where the two ligaments unite, a hollow occurs behind the symphysis pubis, known as the cavum Retzii. This ligament is composed of several layers separated by large veins (the pudendal plexus), which connect the inferior vesical vein with the dorsal vein of the penis and the hypogastric vein.

The sheath of the prostate gland (fascia prostata) is formed by (1) the superior fascia of the urogenital diaphragm on which it lies, (2) by the general pelvic fascia covering the intra-pelvic surfaces of the levatores ani on each side, and (3) it is completed above and behind by the two special layers of pelvic fascia just described. By these means the prostate gland and prostatic urethra are given a firm attachment to the anterior part of the pelvic walls and floor.

In the female an essentially similar arrangement of the pelvic fascia occurs in relation to the vagina and urethra. A crescentic fold of the fascia springs from the pelvic wall in the neighbourhood of the spina ischiadica, and sweeping medially to the lateral fornix of the vagina and in front of the rectum, separates into two layers, posterior and superior. Between the layers are numerous vessels, which, along with the visceral nerves, pierce its free edge. The posterior (recto-vaginal) layer passes medially behind the vagina, and gaining the median plane between the vagina and rectum, gives rise to the anterior wall of the rectal channel, and is attached below to the perineal body in the floor of the pelvis. The superior layer, taking origin from the arcus tendineus, is attached medially to the neck of the bladder, and constitutes the lateral pubo-vesical ligament. It is continuous in front with the anterior pubo-vesical ligament, which, as in the male, is divisible into several layers separated by veins. An intermediate (urethro-vaginal) layer of the fascia passes between and separates the urethra and vagina.

The urethra and vagina are by means of these layers of fascia firmly bound to the pelvic walls and floor, while the uterus and bladder are free to distend in the pelvic cavity.

MUSCLES OF THE PELVIS.

Diaphragma Pelvis.-The pelvic diaphragm is formed by the levator ani and coccygeus muscles, which serve to uphold the pelvic floor, and are related to the rectum and the prostate gland or vagina.

M. Levator Ani.-The levator ani arises from (1) the inferior part of the posterior surface of the body of the pubis, (2) the general pelvie fascia above or along the arcus tendineus, and (3) the pelvic surface of the spine of the ischium.

Its fibres are directed downwards and backwards, to be inserted into (1) the central point of the perineum (perineal body), (2) the external sphincter around

the origin of the anal canal, (3) the ano-coccygeal raphe behind the anus, and (4) into the sides of the lower coccygeal vertebræ.

The levator ani muscle fills up and completes the pelvic floor on each side of the median plane. Enclosed in a sheath derived from the general pelvic fascia along the arcus tendineus, the muscle presents an upper concave surface in relation to the pelvic cavity, prostate gland (or vagina), and rectum, and an

PUBO

COCCYGEUS

SPHINCTER ANI
EXTERNUS

FIG. 437.-THE FASCIAL AND MUSCULAR WALL OF THE PELVIS AFTER REMOVAL OF PART OF THE LEFT HIP BONE.

inferior convex surface which appears in the perineum and forms the medial wall of the ischio-rectal fossa.

The levator ani is divisible into four parts-puborectalis, pubococcygeus, iliococcygeus, and iliosacralis. The puborectalis (levator prostata) is the part inserted into the central point of the perineum. The pubococcygeus is the part inserted into the anus and the ano-coccygeal raphe, and the iliococcygeus and ischiococcygeus are represented by the fibres attached to the sacrum and coccyx. The first two are best developed; the last two series of fibres are the most rudimentary. These several parts of the muscle represent the remains of the flexor caudæ of tailed animals.

Nerve-Supply. The levator ani is supplied from two sources: by the perineal (muscular) branch of the pudendal nerve, and, on its pelvic surface, by special branches from the third and fourth sacral nerves.

Actions. (1) The levator ani muscle serves to uphold and slightly raise the pelvic floor. (2) It is likewise capable of producing slight flexion of the coccyx. (3) The anterior fibres of the levator ani, in the female, sweeping round the vagina, compress its walls laterally, and along with the sphincter vaginæ, help to voluntarily diminish the lumen of the tube. (4) The same part of the muscle in the male elevates the prostate gland (levator prostata). (5) The chief action of the levator ani is in defecation. Along with the external sphincter it acts as a sphincter of the rectum, closing the anal canal. During defæcation the muscle draws upwards the anus over the fæcal mass, and so assists in its expulsion. (6) In parturition, in the same way, the muscle, contracting below the descending foetal head, retards delivery. Contracting on the fœtal head, it draws upwards the pelvic floor over the fœtus, and so assists delivery.

M. Coccygeus.-The coccygeus is a rudimentary muscle overlapping the posterior border of the levator ani. It arises from the ischial spine and the sacrospinous ligament.

It is inserted into the sides of the lower two sacral and upper two coccygeal vertebræ. The muscle is in contact by its anterior border with the levator ani. It is enclosed in pelvic fascia, assists in forming the pelvic floor, and is in contact laterally with the sacro-tuberous and sacro-spinous ligaments.

Nerve-Supply.-The coccygeus is supplied on its pelvic surface by the third and fourth sacral nerves.

Actions. The muscle is a feeble lateral flexor of the coccyx, and assists the levator ani to uphold the pelvic floor.

THE DEVELOPMENT AND MORPHOLOGY OF THE SKELETAL

MUSCLES.

The mesoderm on either side of the embryonic medullary tube separates into three main parts-the myotome, nephrotome, and sclerotome or lateral plates (somatopleure and splanchnopleure).

The myotomes are probably directly or indirectly the source of the striated muscles of the whole body. Each consists at first of a quadrilateral bilaminar mass, resting against the medullary tube and notochord on either side. The cleft between its two layers represents the remains of the cœlomic cavity. In the early stages of embryonic life the growth of the myotome is rapid. On its medial side masses of cells arise, which grow medially and surround the medullary tube and notochord to form the foundation of the vertebral column. On its lateral side cells appear to be given off which participate in the formation of the cutis vera. At the same time the dorsal and ventral borders of the myotome continue to extend, and present extremities (growing points) with an epithelial structure for a considerable period. On the dorsal side it overlies the medullary tube, and gives rise to the muscles of the back; while by its ventral extension, which traverses the somato-pleuric mesoderm in the body wall, it produces the lateral and ventral muscles of the trunk. By a medial extension it probably gives rise also to the hypaxial muscles of the neck and loin. The cells of the medial layer of the myotome are responsible for the formation of the muscle fibres. The cells elongate in a direction parallel to the long axis of the embryo, and give rise, by fusion with the cells of neighbouring myotomes, to the columns and sheets of muscles of the back and trunk. For the most part (e.g. back and abdomen) the originally segmental character of the muscular elements is lost by the more or less complete fusion of adjacent myotomes. The intercostal muscles, however, are the direct derivatives of individual myotomes.

Muscles of the Limbs.-In fishes and (doubtfully) reptiles there is evidence that the myotomes are concerned in the formation of the limb-muscles by their extension into the limb-bud in a manner similar to that described for the trunk. In birds and mammals, however, in which the limb-bud arises as an undifferentiated, unsegmented mass of mesodermic tissue, partly from the mesoderm surrounding the notochord, and partly from the somato-pleuric mesoblast, the myotomes stop short at the root of each limb, and do not penetrate into its substance. Instead, the muscular elements of the limb take origin independently as double dorsal and ventral strata of fusiform cells on the dorsal and ventral surfaces of the limb-bud. These strata are unsegmented; they are grouped around the skeletal elements of the limb, and they gradually become differentiated into the muscle masses and individual muscles of the limb.

Muscles of the Head.-Notwithstanding the obscurity and complexity of this