56 THE MUSCULAR SYSTEM OF THE FROG 



A. Muscles of the Trunk. 

 1. Muscles of the ventral body-wall. 



Pin out the frog on its back, remove the skin, and dean t/te 

 muscles. 



i. The rectus abdominis runs longitudinally along 

 the mid-ventral wall, the muscles of the two sides 

 being separated from each other in the median 

 plane by the linea alba, a longitudinal band of 

 connective tissue, immediately dorsal to which 

 lies the anterior abdominal vein. Each rectus 

 muscle is divided into bellies by five transverse 

 tendinous intersections. 



The muscle arises from the pubes, runs for- 

 wards and is inserted into the dorsal surface of 

 the sternum and coracoid. 



ii. The pectoralis is a large fan-shaped muscle, con- 

 sisting of a thoracic portion, which arises from 

 the whole length of the ventral surface of the 

 sternum ; and an abdominal portion, arising from 

 the aponeurosis along the outer side of the 

 rectus abdominis almost as far back as the 

 pubes. From this extensive origin the fibres 

 converge to the deltoid ridge of the humerus, 

 into which they are inserted, the line of insertion 

 extending down almost to the elbow. 



iii. The obliquus externus is a thin sheet of muscle 

 which arises from the aponeurosis of the back, a 

 short distance from the vertebral column, and 

 covers the whole of the side of the body, the 

 fibres running obliquely downwards and back- 

 wards to end in an aponeurosis which passes 

 dorsal to the rectus abdominis to be inserted 

 into the linea alba. 



iv. The obliquus internus lies beneath the obliquus 

 externus, which must be removed in order to 

 see it. It arises from the transverse processes of 

 the vertebrae from the fourth backwards, and 

 from the ilium. The fibres run downwards and 

 forwards, and are inserted in front into the 

 coracoid and sternum : some of the fibres sur- 

 round, and are inserted into, the rcsophagus and 



