iv THE INTERNAL STRUCTURE 73 



ment of old teeth by new throughout most of the life of the 

 animal, the process ceasing only in old individuals. The 

 walls of the old teeth become partly absorbed by means of 

 large multinucleate cells, the osteoclasts ; in this way they 

 become freed from their attachment to the jaw bone, and 

 are then cast out. New teeth are produced below the old 

 ones, whose place they finally take. 



Organs in the Body Cavity. — If the ventral body wall of 

 the frog be cut through and the cut edges be spread apart 

 and pinned down, there will be opened up a large cavity 

 containing the principal internal organs of the body. This 

 space is called the body cavity, or ccelom. It lies ventral to 

 the vertebral column, or backbone, which may be seen when 

 the internal organs are pushed aside. If the middle part 

 of the pectoral girdle, or bony support of the fore limbs, is cut 

 away, the exposure of the parts will be made more complete. 

 Near the anterior end of the body cavity lies the heart, not 

 on the left side of the body, as in ourselves, but very nearly 

 in the middle line. It is inclosed in a transparent sac, the 

 pericardium, through which one may see the two auricles, 

 which are thin-walled and appear dark red from the blood 

 they contain, and a posterior cone-shaped division, the 

 ventricle, which has a very thick muscular wall and is of a 

 pink or light reddish color. The pericardium is united to 

 the ventral body wall by a thin sheet of membrane, the 

 posterior edge of which is free and incloses the anterior 

 abdominal vein, which runs along the mid-ventral line and 

 finally empties into the liver. 



The liver is a large, reddish brown organ lying above and 

 partially surrounding the pericardium ; it consists of three 

 lobes, — a median, a right, and a left. On the ventral side 

 of the median lobe lies the gall bladder, which is connected 

 with the intestine by means of the gall duct. Projecting 



