NECTURUS 79 



clavian also gives off near its base the cutaneous artery, which 

 passes back in the body wall near the midventral plane and sup- 

 plies the skin with blood; the outer ends of the spinal arteries 

 anastomose with it. 



The next large artery to leave the aorta is the gastric artery, 

 which supplies the stomach. Near the center of the abdominal 

 cavity the large cceliaco-mesenteric artery leaves the aorta and 

 goes to the stomach, spleen, and intestine. The intestine is also 

 supplied with blood by the posterior mesenteric arteries, which are 

 about twenty in number and branch off from the hinder portion 

 of the aorta. In most of the higher vertebrates a single posterior 

 mesenteric trunk is present. The numerous genital arteries and 

 renal arteries also branch off from the hinder portion of the aorta 

 and supply the genital organs and the kidneys. The large iliac 

 arteries leave the hinder end of the aorta and go to the hind legs. 



Exercise 16. Draw a semidiagrammatic sketch of the entire arterial 

 system. 



The Muscular System. Remove the skin from the entire body 

 of a Necturus. Inasmuch as the skin adheres very tightly to 

 muscles beneath, care must be taken not to cut the latter, especially 

 on the lower jaw and the breast. Begin with a median incision 

 down the back of the animal, and skin toward the ventral side. 



Observe the great complex muscle which forms the side of 

 the trunk and the tail. It will be seen to be made up of a 

 succession of muscle segments which are called myotomes or 

 myomeres and are separated from one another by connective- 

 tissue septa called myocommas. Each myotome is a plate between 

 two myocommas and consists of parallel muscle fibers which run 

 across from one myocomma to the other. The body musculature 

 of the trunk and the tail is essentially like that of the fish. In the 

 midventral line is a white line, the linea alba, which divides the 

 muscles on the right side from those on the left. 



In the region of the head, the legs, and the arms specialized 

 muscles are present. The entire ventral surface of the lower jaw 

 is bounded by a broad, thin muscle, the submandibular, the fibers 

 of which have a transverse direction and are divided by a me- 



