COMPARATIVE ANATOMY OF THE MUSCULAR SYSTEM 137 



external oblique; a middle internal oblique; and an internal layer, the transversus 

 abdomiuus. 



a) External oblique: This is the outermost of the muscle layers of the ab- 

 dominal wall. It was described above. 



b) Internal oblique: Very carefully cut through the middle of the belly of 

 the external oblique, in a longitudinal direction, and separate it from the under- 

 lying muscle, which is the internal oblique. This separation is often difficult. 

 The internal oblique is a short muscle lying beneath the more dorsal portion of 

 the external oblique. Its fibers are directed obliquely downward and forward, 

 and are continued by a very broad aponeurosis. Origin: rabbit— second sheet of 

 the lumbodorsal fascia, posterior ribs and inguinal ligament; cat — second sheet of 

 the lumbodorsal fascia and border of the pelvic girdle. Insertion, on the linea 

 alba by the extensive aponeurosis; action, compressor of the abdomen. 



c) Transverse: On cutting through the preceding and separating the edges, 

 a third muscle layer, very thin, will be found. This is the transversus abdominis. 

 Its fibers are directed ventrally and slightly posteriorly. Origin, insertion, and 

 function similar to the preceding. 



d) Rectus abdominis: This is a long slender muscle on each side of the linea 

 alba, extending from the pubic symphysis to the anterior part of the thorax. 

 It is found inside of and between the aponeuroses of the preceding muscles. 

 Slit open these aponeuroses along each side of the linea alba and expose the rectus 

 abdominis. Its fibers run longitudinally and in the cat are crossed at regular 

 intervals by transverse white lines. Origin, anterior end of the pubic symphysis; 

 insertion, sternum and costal cartilages; action, retracts ribs and sternum 

 and constricts the abdomen. Underneath the transverse and the rectus ab- 

 dominis is the peritoneal membrane, or lining of the coelom. 



The foregoing muscles are hypaxial muscles, formed into sheets by the side- 

 wise fusion of myotomes. The rectus abdominis represents the ventral ends of 

 the myotomes. The three layers on the sides of the abdomen have resulted 

 from a splitting process. 



4. The epaxial muscles. — Remove the lumbodorsal fascia over the posterior 

 part of the back, finding beneath it a great thick mass of muscle inclosed in a 

 tough shining fascia. Note attachment of the lumbodorsal fascia in the median 

 line to the neural spines of the vertebrae. The mass of muscle is divisible 

 into a slender narrow median portion, the multifidus, next to the median dorsal 

 line, and a very thick lateral portion, the sacraspinalis. The latter is in the cat 

 readily divisible into three longitudinal portions. These muscles continue up 

 into the thoracic region, where they will be seen later, to the back of the head, 

 their various portions receiving different names. They are the epaxial muscles 

 and are sharply separated from the hypaxial muscles in the abdominal region 

 by a furrow which corresponds to the position of the horizontal skeletogenous 

 septum of fishes. These muscles are the most powerful muscles in the body. 



