412 THE MUSCULAR SYSTEM. 
The ischio-coccygeus is a rudimentary muscle overlapping the posterior border 
of the levator ani. It arises from the ischial spine and the small sacro-sciatic 
ligament, and is inserted into the sides of the lower two sacral and upper two 
coccygeal vertebre. The muscle is separated from the rectum by the visceral 
pelvic fascia, and is in contact externally with the sacro-sciatic ligaments. 
NERVE SUPPLY. 
The levator ani is supplied by two nerves: by the perineal (muscular) branches of the pudic 
nerve, and, on its pelvic surface, by special branches from the third and fourth sacral nerves. 
The ischio-coccygeus is supplied on its pelvic surface by the third and fourth sacral nerves. 
ACTIONS. 
(1) The levator ani and ischio-coccygeus serve to uphold and slightly raise the pelvic floor. 
(2) They are 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 vagine, help to voluntarily diminish the lumen of the tube. (4) The 
same part of the muscle in the male elevates the prostate gland (levator prostate). (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 defecation the muscle draws upwards the anus 
over the fecal mass, and so assists in its expulsion. (6) In parturition, in the same way, the 
muscle, contracting below the descending feetal head, retards delivery. Contracting on the feetal 
head, it draws upwards the pelvic floor over the foetus, and so assists delivery. 
THE DEVELOPMENT AND MORPHOLOGY OF THE SKELETAL MUSCLES. 
Our knowledge of the development and morphology of the muscular system is very 
incomplete. It has already been shown, in the chapter on general embryology, that the 
ana 
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{NIC fee Diy SCHEME TO ILLUSTRATE 
veg / 1. Z THE DISPOSITION OF THE 
MYOTOMES IN THE Em- 
BRYO IN RELATION TO 
THE HEAD, TRUNK, AND 
LIMBS. 
A, B, C, First three cephalic 
myotomes ; N, 1, 2, 3, 4, 
Last persisting cephalic 
myotomes; C., T., L., S., 
Co., The myotomes of the 
cervical, thoracic, lumbar, 
sacral, aud caudalregions ; 
Te, DT. DN Ves Viesmvalee 
VIL, VILL xyes eee 
XIIL., refer to the cranial 
nerves and the structures 
with which they may be 
embryologically — associ- 
ated. 
WW. A 
mesoblast on either side of the embryonic medullary tube separates into three main parts— 
the myotome, nephrotome, and 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 
