364 PHYSIOLOGY. 



effects could also be called out in a reflex manner. This explains 

 asthmas due to reflex irritations transmitted to the centers of the vagi. 

 Atropine and lobeline paralyze the vagus ending in the bronchial 

 muscles. This explains their utility in spasmodic asthma. 



VARIOUS FEATURES OF RESPIRATION. 



Nasal Breathing. During ordinary, quiet breathing most peo- 

 ple breathe through the nostrils, keeping the mouth closed. This 

 is very proper and there are certain advantages to be derived by so 

 doing. Thus, in the passage of the air through the nostrils, whose 

 walls are narrow and somewhat tortuous, the air is not only warmed, 

 but rendered moist as well. By this means there is prevented the 

 irritation occasioned by cold, dry air upon the lining mucous mem- 

 brane. In addition, the smaller foreign particles are caught by the 

 mucous lining and carried outward by the instrumentality of the 

 ciliated epithelium. 



Pathological. Pulmonary cedema, which is a transudation of 

 lymph into the pulmonary alveoli, occurs (1) when there is very great 

 resistance to the blood-stream in the aorta and its branches; (2) 

 when the pulmonary veins are occluded ; ( 3 ) when the left ventricle, 

 owing to mechanical injury, ceases to beat, while the right ventricle 

 continues in its contraction. 



Injection of muscarine rapidly produces pulmonary cedema by 

 reason of increased pressure and slowing of the blood-stream in the 

 pulmonary capillaries. The effects of this drug are counteracted by 

 atropine. 



Eelation of Respiration to the Nervous System. Movements of 

 respiration are entirely dependent upon the nervous system. They 

 are nicely balanced actions, performed by voluntary muscles under 

 the guidance of a special presiding nerve-center, namely : the respira- 

 tory center. Through its influence the muscles of inspiration and 

 expiration are kept working rhythmically and regularly, whether the 

 individual be awake or sleeping. Co-ordinated impulses are con- 

 stantly proceeding from the center to the muscles involved. How- 

 ever, the muscles being voluntary, they may be controlled momen- 

 tarily by the will, and respiration be made to cease entirely for a 

 minute or two. Soon the excess of carbon dioxide becomes so great 

 that the will is overcome and respiration is begun again under the 

 supervision of the respiratory center. 



The Respiratory Center. This center is located in the medulla 

 oblongata, in the formatio reticularis, behind the superficial origin 



