i8 AN INTRODUCTION TO ZOOLOGY 



stout tube runs forwards between the auricles and then divides up 

 into two trunks, passing right and left. This is the truncus arteriosus, 

 and from it spring the arteries or blood-vessels that carry the blood 

 from the heart all over the body. If the heart be turned to one side 

 a triangular very thin-walled sac, the sinus venosus, will be seen. 

 Into this open large blood-vessels, the veins, bringing the blood back 

 from the whole of the various parts of the body. A network of blood- 

 vessels will be seen ramifying all over the viscera, and they are. 

 termed arteries or veins, according to whether they convey blood 

 away from or towards the heart. 



Just behind the heart are two large reddish-brown masses, one 

 on each side. These are the two parts or lobes of the liver, and are 

 joined together in the middle line at the front. Between them will 

 be seen a small thin-walled sac, the gall-bladder, usually filled with 

 a dark green liquid, the gall or bile, which is made by the liver and 

 utilised in digestion. On the dorsal side of the liver and heart will 

 be seen two bright pink sacs with honeycombed walls ; these are the 

 lungs. By inserting a blowpipe into the glottis they can quite 

 easily be inflated and rendered conspicuous. 



A large whitish tube will be seen on the left behind the liver. 

 This is the stomach, and if traced forward dorsal to the liver it 

 will be found to merge into a slightly narrower tube, the gullet or 

 oesophagus, which in its turn opens into the pharynx. The line 

 of demarcation between the stomach and oesophagus is not nearly 

 so sharp in Rana as it is in the rabbit or ourselves. The posterior 

 end of the stomach is marked off by a slight constriction, the pylorus, 

 from a very long narrow tube, the small intestine, the first part of 

 which, lying more or less parallel with the stomach, is called the 

 duodenum, and the remainder the ileum. This pursues a twisted 

 course, and finally expands to form a wide tube, the large intestine 

 or rectum, which communicates with the exterior through the cloaca. 

 Thus we find that the food is passed from the mouth into a tube, the 

 enteric or alimentary canal, which has no openings save the mouth 

 and cloaca. In order that the food may be distributed to the 

 various organs it is necessary for it to pass through the wall of this 

 canal. This is accomplished by its being made soluble or digested. 



A small yellow elongated mass is situated between the duodenum 

 and the stomach, this is the pancreas, and it also is concerned with 

 digestion. 



Near the front end of the rectum is the spleen, a globular structure 

 about the size of a small pea and dark red in colour. 



The urinary bladder is a fairly large bilobed sac with very thin 

 walls, and is to be found opening into the ventral wall of the cloaca 

 and lying in the posterior part of the body cavity. 



