80 ESSENTIALS OF ZOOLOGY 



Between the skin and muscle are a series of subcutaneous lymph 

 sacs; other sinuses are in the mesenteries, around the vertebral 

 column, and elsewhere. The peritoneal and pericardial cavities are 

 connected with the Ijmiphatic system. Nephrostomes on the ventral 

 surface of the kidney convey lymph from the peritoneal cavity into 

 the renal veins. 



Respiratory Organs and Respiration 



Air enters through the nostrils, passes into a small olfactory 

 chamber and then into the mouth cavity through the internal nares, 

 which open in the roof of the mouth. The mouth is kept tightly 

 closed in breathing. Air is sucked in by lowering the floor of the 

 mouth -and is then forced into the lungs by raising the floor, the 

 external nares being closed by valves. This pushes the air through 

 the slitlike glottis immediately behind the tongue in the floor of the 

 mouth, thence into a short larynx which connects with the lungs. 



The wall of the larynx is reinforced by a framework of cartilage, 

 and the laryngeal chamber supports two horizontal fleshy folds, the 

 vocal cords, which extend across the passageway. When a frog 

 croaks, its mouth and nostrils are kept tightly closed, and the air 

 is forced back and forth between lungs and mouth cavity, causing 

 the vocal cords to vibrate. 



The two lungs lie dorsal to the heart on each side and dorsal to 

 the liver. They are very elastic sacs with their inner walls raised 

 into a number of ridges, forming chambers which are called alveoli. 

 These chambers are richly supplied with a network of blood vessels 

 for facilitating the oxygenation of the blood. In the bullfrog the 

 lungs are also important as a hydrostatic organ. 



While the lungs play the major role in respiration, other factors 

 are of considerable importance. The lining of the mouth of the bull- 

 frog contains a large number of blood vessels and serves for a type of 

 respiration known as hucco-pharyngeal respiration. With the glottis 

 closed, air is drawn into the mouth cavity and forced out by rhythmi- 

 cal movements of the throat. Oxygen is taken up by blood vessels in 

 the lining of the mouth by diffusion. 



The skin of the bullfrog plays a large part in its respiration, and 

 frogs that are not protected from drying out soon die. Gaseous ex- 

 change of carbon dioxide and oxygen can take place through the 

 moist vascular skin, and is known as cutaneous respiration. During 



