BODY-WALL 



19 



on the top of the head and the front of the shanks— there 

 are no muscles, and the bones are covered only by skin and 

 connective tissue. 



Passing along the middle line of the belly is a dark 

 longitudinal streak (Fig. 2, ahd. v) : this is a blood-vssel, the 



oc.st 



Fig. 2. — A frog with the skin {sk) of the ventral surface cut through and turned 

 back right and left, so as to expose the muscles. Of these the mylohyoid {my. /ly), 

 pectoralis {,pct)^ external oblique {ext. odl), and rectus abdominis^ {7-ct. aba) 

 are lettered. On the right side (left in the figure), the posterior portion of the 

 pectoral muscle is cut away, its two ends {pci' fct") only being left. The 

 cartilaginous extremity of the breast-bone (xiphlsternum, x. si) is shown, as 

 well as the abdominal {abd. v), musculo-cutaneous {in. c. r') and subclavian 

 {scl. v) veins, and the cutaneous artery (c. a). 



abdominal vein. On each side of the body another vein {m. 

 c. v) is seen forming a loop, one limb of which is on the 

 turned-back flap of skin, while the other passes between 

 the muscles not far from the armpit : this vessel is the 

 musculo-cutaneous vein. Both these veins, and many others 



