MORPHOLOGY OF THE SKELETAL MUSCLES. 495 



Actions. (1) Tli_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 



f theTevator ani, in tlie female, sweeping round The vagina, compress its walls laterally, and 



I long with the sphincter vaginae, help to voluntarily diminish the lumen of the tube. (4) The 



ame part of the muscle in the male elevates the prostate gland (levator prostatse). (5) Thp p."hiqf 



i ction of the levator, ani is in defalcation. Along with the external sphincter it acts as a sphincter 



Ffn rectum,"closmg the anal caSiF During defecation the muscle draws upwards the anus 



ver the faecal mass, and so assists in its expulsion. (6) 'In parturition, in the same way, the 



auscle, contracting below the descending foetal head, retards delivery. Contracting on the foetal 



lead, it draws upwards the pelvic floor over the foetus, and so assists delivery. 



M. Coccygeus. The coccygeus is a rudimentary muscle overlapping the 

 josterior border of the levator ani. It arises from the ischial spine and the sacro- 

 pinous ligament. 



It is inserted into the sides of the lower two sacral and upper two coccygeal 

 ertebrse. 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 

 ontact laterally with the sacro-tuberous and sacro-spinous ligaments. 



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

 acral nerves. 



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

 .phold the pelvic floor. 



[E DEVELOPMENT AND MORPHOLOGY OF THE SKELETAL 



MUSCLES. 



The mesoderm on either side of the embryonic medullary tube separates into three 

 iiain parts the myotome, nephrotome, and sclerotome or lateral plates (somatopleure 

 nd splanchnopleure). 



The myotomes are probably directly or indirectly the source of the striated muscles 

 f the whole body. Each consists at first of a quadrilateral bilaminar mass, resting 

 .gainst the medullary tube and notochord on either side. The cleft between its two layers 

 epresents the remains of the ccelomic cavity. In the early stages of embryonic life the 

 ;rowth of the myotome is rapid. On its medial side masses of cells arise, which grow 

 aedially and surround the medullary tube and notochord to form the foundation of the 

 ertebral column. On its lateral side cells appear to be given off which participate in the 

 ormatioii of the cutis vera. At the same time the dorsal and ventral borders of the 

 ayotome continue to extend, and present extremities (growing points) with an epithelial 

 tructure for a considerable period. On the dorsal side it overlies the medullary tube, and 

 , ives rise to the muscles of the back ; while by its ventral extension, which traverses the 

 omato-pleuric mesoderm in the body wall, it produces the lateral and ventral muscles of 

 he trunk. By a medial extension it probably gives rise also to the hypaxial muscles of 

 he neck and loin. The cells of the medial layer of the myotome are responsible for the 

 ormation of the muscle fibres. The cells elongate in a direction parallel to the 

 ong axis of the embryo, and give rise, by fusion with the cells of neighbouring myotomes, 

 o the columns and sheets of muscles of the back and trunk. For the most part (e.g. back 

 nd abdomen) the originally segmental character of the muscular elements is lost by the 

 acre or less complete fusion of adjacent myotomes. The intercostal muscles, however, 

 re the direct derivatives of individual myotomes. 



Muscles of the Limbs. In fishes and (doubtfully) reptiles there is evidence that 

 he myotomes are concerned in the formation of the limb-muscles by their extension into 

 he limb-bud in a manner similar to that described for the trunk. In birds and mammals, 

 lowever, in which the limb -bud arises as an undifferentiated, unsegmented mass of 

 aesodermic tissue, partly from the mesoderm surrounding the notochord, and partly from 

 'he somato-pleuric mesoblast, the myotomes stop short at the root of each limb, and 

 3 not penetrate into its substance. Instead, the muscular elements of the limb take 

 rigin independently as double dorsal and ventral strata of fusiform cells on the dorsal and 

 entral surfaces of the limb-bud. These strata .are unsegmented ; they are grouped around 

 he skeletal elements of the limb, and they gradually become differentiated into the muscle 

 Basses and individual muscles of the limb. 



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



