THE FROG 79 



by a dissection of them in the animal, and a thorough study of the draw- 

 ings. 



A. MUSCLES OF THE TRUNK 



1. Muscles of the lower or ventral side. 



(a) Muscles of the abdomen. 



e. g. Rectus abdominis, a wide band running along the abdo- 

 men divided lengthwise down the middle by the connective tissue linea 

 alba and transversely by tendinous intersections. 



Obliquus externus, a broad sheet at each side of the body, 

 arising from an aponeurosis known as the dorsal fascia which covers 

 the muscles of the back, and inserted into the linea alba above the rectus 

 abdominis. 



Obliquus internus and transversus, muscular sheets below the 

 external oblique. 



By their contraction all these muscles lessen the size of the body 

 cavity and compress the organs within it. 



Innervation : All of these muscles are innervated by twigs from 

 IV, V, VI and VII spinal nerves. 



(b) Muscles of the Breast Region. 



e. g. Pectoralis, large and fan-shaped, inserted into the deltoid 

 ridge of the humerus and consisting of a sternal portion which arises 

 from the pectoral girdle, and an abdominal portion which arises from 

 the aponeurosis at the side of the rectus abdominis. 



It draws down the arm. 



Innervation : Twig from II spinal nerve. 



Coraco-radialis, arising from the coracoid and inserted into the 

 upper end of the radius. It bends the arm. 



Innervation: Twig from II spinal nerve. 



2. Muscles of the Back. 



(a) Muscle inserted into the lower jaw. 



Depressor mandibulae, triangular, arising from the supra- 

 scapula and inserted into the angle of the lower jaw, which it draws 

 downwards and backwards, thus opening the mouth. 



(b) Muscles inserted on the fore-limb. 



e. g. Latissimus dorsi ( ) triangular, aris- 



ing from the dorsal fascia and inserted into the deltoid ridge. It draws 

 back the arm. 



Infraspinatus, in front of and similar to the latissimus dorsi. It 

 raises the arm. 



Innervation : Twig from II spinal nerve. 



(c) Muscles inserted into the shoulder girdle. 



e. g. Levatur anguli scapulae, arising from the skull and in- 

 serted into the under side of the suprascapula, which it draws forward. 

 Innervation : Twig from I spinal nerve. 



