BODY-WALL 



19 



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, abd. v) : this is a blood-vessel, 

 the abdominal vein. On each side of the body another 



sJt 



pel 



cc.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. hy), 

 pectoralis (pet), external oblique (ext. obi), and rectus abdominis (ret. abd) 

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

 pectoral muscle is cut away, its two ends (pet', pet") only being left. The 

 cartilaginous extremity of the breast-bone (xiphisternum, x. st) is shown, as well 

 as the abdominal (abd. v), musculo-cutaneous (m. c. v), and brachial (scl. v) 

 veins, and the cutaneous artery (c. a). (Natural size.) 



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 which will be seen in the course of the 



C 2 



