RESPIRATION 55 



cells. Microscopic blood vessels are placed everywhere between 

 alveoli and in their walls. Thus, we have in effect a thin sheet 

 of blood separated from a thin sheet of air by the thinnest pos- 

 sible layer of animal membrane. 



Lungs. — There are two, right and left. These are the essential 

 organs of respiration, and are located in the thoracic cavity. 

 The pleura is a thin serous membrane one layer of which lines 

 the chest cavity. Another layer encloses the lungs, each in a 

 separate sack. Each lung is cone-shaped. In the lungs of a 

 horse the lobes are not distinctly marked. Some authors make 

 no definite divisions; others describe the right lung as having 

 three lobes, and the left tw^o. 



In the cow's lungs the lobes are distinctly marked. The left 

 lung has three distinct lobes. The right lung has four lobes 

 by reason of the anterior lobe being divided into two parts : first 

 and second. 



The bronchi are subdivided until they are very small and are 

 then called bronchioles. Each bronchiole terminates in a very 

 small cavity made by a number of air cells opening together. 

 These cells have very thin walls, and are separated by loose con- 

 nective tissue in which minute blood vessels are located. Blood 

 is then separated from the air by only a very thin membrane. 



Practical application. — The agricultural student who takes 

 "Dressing and Curing Meats" should make observations con- 

 cerning the probable relations between external appearance of 

 the chest and actual chest capacity. This question has fre- 

 quently arisen in connection with discussions on bovine tuber- 

 culosis. Is the external appearance probably a reliable guide as 

 to actual chest capacity? Why or why not? 



Study a skeleton or chart and note the curvature of the ril)s. 

 Bear in mind that the ribs move forward and outward during 

 inspiration. What relation, if any, between length of rib, de- 

 gree of rib curvature and possil)le chest expansion? What type 

 of nostril, nasal chamber, and larynx do you usually find with 

 great lung capacity? 



The working efficiency of every cell in the body, nerve, muscle, 

 secretory gland cell {e.g., milk gland), depends greatly upon its 

 supply of oxygen and the removal of its waste. 



Disorders of the respiratory organs are many and usually re- 

 quire professional treatment. The following are practical ex- 

 amples of respiratory disorders: Catarrh of respiratory mucous 



