The Respiratory System 149 



THE ANATOMY OF THE RESPIRATORY SYSTEM 



In mammals as in all other animals, the essential respiratory device 

 is a thin moist membrane; however, due to the increased bulk of these 

 animals, the constant body temperature, and the increased metabolic 

 rate, many accessory structures for increasing the efficiency of the ex- 

 change are absolutely necessary. 



Air before it can be exchanged with the blood must be warmed, 

 cleaned, and humidified. Special structures developed in the nose take 

 care of this initial problem. Air passes through the nose, the naso- 

 pharynx, the pharynx, and enters the respiratory tube proper, through 

 the glottis. The upper part of the tube is a cartilaginous box, the larynx. 

 Beyond the larynx the tube is known as the trachea. The trachea passes 

 posteriorly to divide into the two bronchi, one of which passes to each 

 lung. Within the lungs, the bronchi branch into the bronchioles and 

 continue branching and rebranching until they terminate in numerous 

 thin-walled sacs, the alveoli. It is within the alveoli that the actual 

 gaseous exchange takes place. 



The Nose. — In mammals, the nose serves two important functions : 

 it serves as a passageway for air and as a sense receptor for odors. 

 Among lower vertebrates such as the fish, the nose has but a single 

 function, that of the detection of odors. The adult terrestrial amphibians 

 are the first to utilize the nose as an air pathway. In them, however, 

 the air passes directly into the mouth. It is among the reptiles that the 

 first real separation of air and food passages occurs. The dorsal nose 

 is separated from the mouth by means of the hard palate, and the in- 

 ternal openings of the nose are pushed far back toward the throat. In 

 the alligator this condition is developed to an extreme. The internal 

 nares are in the throat, and by means of a special membrane at the 

 rear of the mouth, the air passage is effectively sealed off from the 

 mouth. By means of this ingenious device, the alligator can hold its 

 mouth open below water and not drown ! 



In mammals as in reptiles, the hard palate separates the nasal and 

 mouth cavities. The internal nasal openings, or internal nares, open 

 back toward the pharynx. Inasmuch as both mouth and nasal passages 

 open into the pharynx, breathing can be carried on through both routes. 

 There are definite advantages to nasal breathing, however, and mouth 

 breathing usually occurs only when the nasal passages are in someway 

 blocked. 



