4o8 ZOOLOGY FOR MEDICAL STUDENTS chap. 



to a small opening in the skeletal wall enclosing the otocyst. The 

 original communication with the pharynx remains open as the Eustachian 

 tube (E) which serves to keep the air-pressure equal on the two sides 

 of the ear-drum so as to permit of its free vibration. 



The lung-apparatus of the Amphibian consists of a symmetrical right 

 and left lung of relatively small size, lying in the peritoneal cavity and 

 communicating with the pharynx by a ventrally placed glottis. The 

 amphibian method of lung breathing is well seen in an ordinary frog. 

 The mouth is shut, the external nares open, and the floor of the mouth is 

 raised and lowered rhythmically by muscular contraction, an important 

 part being played by the mylo-hyoid — a thin sheet of muscle which 

 passes across from the lower jaw of one side to that of the other. In 

 this way air is passed in and out of the buccal cavity through the olfactory 

 organ. At longer intervals the external nares are closed by a valvular 

 arrangement and now when the floor of the mouth is pressed upwards 

 the glottis opens and the air is forced back through it into the lungs. 

 When pressure is relaxed the air is expelled again through the glottis 

 by the elasticity of the distended lung-wall. 



It may be mentioned parenthetically that the respiratory processes of 

 the adult frog are not confined to the lung. The moist skin of the body 

 is richly supplied with blood and active respiratory exchange takes place 

 through it. The same happens with the vascular lining of the buccal 

 cavity which as has been mentioned is rhythmically filled with fresh air. 

 In some of the Salamanders this buccal respiration has become accentu- 

 ated to such a degree as completely to replace pulmonary breathing and 

 the lungs have in the adult entirely disappeared. 



In Amphibians which possess lungs we find that these differ much in 

 their degree of complexity. In a Newt the lung is a simple thin-walled 

 sac with a rich capillary network — an excellent object for observing 

 capillary circulation through the microscope. The newt is killed 

 by destroying the brain and spinal cord and then carefully opened 

 and laid on a microscope slide in normal salt solution, the lung being 

 drawn out to one side so as to permit the microscope to be focused 

 upon it. 



In a Frog the lining of the lung is no longer smooth but projects out 

 into numerous little recesses or pockets so as to enlarge the area of the 

 respiratory surface. In a Toad (Bufo) the development of these recesses 

 is carried much further, so that the wall of the lung assumes a spongy 

 character. 



In those Amphibians which have a well-developed " neck " region 

 the unpaired portion of lung next the glottis is drawn out into a long 



