MORPHOGENESIS OF MUSCULAR SYSTEM 715 



328B). (See Lewis, W. H., '10.) Between the above two major groups of 

 muscles derived from the epaxial muscle column are other epaxial derivatives 

 such as the semispinalis and multifidus muscles. 



The ventral or hypaxial portions of the myotomes overlying the develop- 

 ing ribs fuse into a continuous mass, while the medial portions of the myo- 

 tomes lying between the ribs give origin to the Mm. intercostales interni 

 and externi. The ventral ends of the fused myotomes on either side of the 

 midventral line split off longitudinally to form the M. rectus abdominis 

 which becomes an elongated sheet, extending from the anterior pectoral area 

 caudal to the differentiating pelvic girdle. The tendency toward segmentation 

 of the two rectus abdominis muscles probably represents a secondary process 

 in man. Tangential splitting of the fused thoracic and abdominal myotomes 

 and migration of the fibers give origin to the Mm. obliquus abdominis ex- 

 ternus, obliquus abdominis internus, transversus abdominis, serratus posterior 

 superior, and serratus posterior inferior. 



The deep or subvertebral muscles below the vertebral column in the dorsal 

 area are derived from two sources. The Mm. longus colli and longus capitis 

 arise from the migration of myotomic tissue to the ventral vertebral surfaces 

 in the neck region, whereas the Mm. iliopsoas appear to be derived from the 

 musculature of the hind limb (Lewis, W. H., '10). 



d) Muscles of the Cloacae and Perineal Area. The muscle tissue of 

 the cloaca forms a circle of constricting muscular bands which surround the 

 cloacal opening. These muscular bands are derived from myotomic tissue of 

 the posterior truncal region. 



In the higher mammals, the primitive cloacal opening becomes divided 

 during development into anterior urogenital and posterior anal openings, and 

 the cloacal musculature is divided into the musculature associated with the 

 urethra, external genital structures, and the anal sphincter (fig. 329C). 



e) Development of the Musculature of the Tail Region. The 

 musculature of the tail arises from the tail-bud mesoderm of the early embryo. 

 This mesenchyme condenses to form myotomic concentrations which later 

 divide into epaxial and hypaxial segments as in the truncal region of the 

 body. These myotomic segments are well developed in all fishes and in the 

 adults of amphibia other than the Anura. In fishes the enlarged condition 

 of the epaxial and hypaxial muscles of the tail region coincides with the 

 elongation of neural spines and hemal processes of the tail vertebrae where 

 they serve the functidn of moving the caudal fin from side to side. Three 

 main types of caudal fin skeletal arrangement in fishes (see fig. 331B-D) 

 act as the framework for the fin which serves the relatively enormous pro- 

 pulsive force generated by the tail musculature. 



In Necturus, in Cryptobranchus, and in other water-dwelling amphibians, 

 and also in crocodilians, whales, etc., the tail musculature is developed to 

 serve the natatorial function which requires a lateral movement of the tail. 



