INNERVATION OF THE RESPIRATORY MOVEMENTS. 709 



inhibition or excitation through sensory nerve fibers and variations 

 in the carbon dioxid of the blood, it is to be borne in mind that this 

 irritable center may be influenced in many other ways, for example, 

 by the specific action of various drugs or toxins, by variations in the 

 inorganic elements of the blood, etc. 



Innervation of the Bronchial Musculature. Numerous in- 

 vestigators, using different methods, have demonstrated that the 

 bronchial musculature is supplied through the vagus with motor and 

 inhibitory fibers, bronchoconstrictor and bronchodilator fibers, as 

 they are usually called.* Stimulation of the constrictors causes a 

 narrowing of the bronchi, and therefore increases the resistance to 

 the inflow and outflow of air. Some observers state that these fibers 

 are normally in a condition of tonic activity (Roy and Brown), but 

 others find little evidence for this belief. An artificial tonus 

 that is, a condition of maintained activity of the constrictor fibers- 

 may be set up by the action of those drugs (muscarin, pilocarpin, 

 etc.), which are known to stimulate the endings of bulbar auto- 

 nomic nerve-fibers. Their effect is removed by the action of atro- 

 pin. These fibers are stimulated also during the excitatory stages 

 of asphyxia. Reflex stimulation of the constrictors is obtained 

 most readily (Dixon and Brodie) by irritation of the nasal mucous 

 membrane, and it seems probable that in bronchial or spasmodic 

 asthma these fibers are also stimulated reflexly. 



The normal conditions under which the constrictors and dilators 

 are brought into play can scarcely be stated. Irritating vapors or 

 even C02 lead to a bronchoconstriction, and this reflex, as stated on 

 p. 699, may be regarded as protective. When a constriction of the 

 bronchial musculature exists it may be abolished by the paralyzing 

 action of atropin, or temporarily by injections of extracts of 

 lobelia, or by the anesthetic effect of inhalations of chloroform or 

 ether. Nicotin also causes a dilatation. 



* For references to literature, see Dixon and Brodie, "Journal of Physi- 

 ology/' 29, 97, 1903., 



