THE FROG 57 



Each lung is an oval sac with a pointed posterior end, with extremely 

 elastic walls, and usually is found in a collapsed condition. It may, 

 however, be inflated easily by means of a blowpipe inserted into the 

 larynx. The interior is divided up by a network of partitions into a 

 number of chambers incomplete at their inner ends. These in their 

 turn are sub-divided into a larger number of smaller cells, the 

 alveoli, by a multitude of smaller partitions. 



The act of breathing is somewhat complex, and differs from 

 that in man. The frog possesses no ribs whereby it can increase 

 the size of the chest cavity and so cause the air to rush in ; on the 

 contrary, the lungs are highly elastic and tend to expel the air, which, 

 therefore, has to be forced into them. The forcing is carried out by 

 the floor of the mouth in conjunction with certain valves in the 

 nostrils. Internal and external nares are connected by a continuous 

 passage, allowing of easy ingress and egress of air when not closed 

 by the valves. Three stages can be recognised in the process : 

 firstly, aspiration, during which air is drawn in through the nostrils 

 by means of lowering the floor of the mouth, the glottis being kept 

 closed ; secondly, expiration, when the contraction of the trunk 

 muscles and the elasticity of the lungs expel some of the air from the 

 lungs through the glottis, now open, into the buccal cavity ; lastly, 

 immediately following upon the foregoing, inspiration, in which the 

 mixed air is pumped into the lungs by raising the floor of the mouth 

 and at the same time keeping the nares closed. It is obvious from 

 this that the air breathed into the lungs is not fresh, but a mixture 

 of pure air taken in through the nostrils with impure air that has 

 already been in the lungs. If a living frog is observed it will be 

 noticed that the floor of the mouth is constantly being moved up and 

 down, these movements being concerned with respiration. 



The lungs are extremely well supplied with blood-vessels whose 

 smallest branches ramify in a close network in the walls of the 

 alveoli. They are covered only by the alveolar epithelium, which is 

 composed of a single layer of flattened cells, and so the blood in 

 them is only separated from the air by their own very thin walls 

 and the alveolar epithelium. Thus the oxygen in the air is enabled 

 to diffuse into the blood, where it forms an unstable compound 

 with the colouring matter of the blood, and in this condition is 

 carried to the tissues, where it is utilised to release energy by the 

 oxidation of certain substances. In this way the carbonic acid gas, 

 one of the main products of the oxidation process, that has been 

 collected up from the various parts and dissolved in the fluid portion 

 of the blood is able to pass from it into the air in the lungs, whence it 

 is expelled. 



This taking in of oxygen from the air and giving off of carbonic 



