THE EFFERENT NERVES 323 



loud, sharp, aspirating sound in which h or cli is the predominating component. 

 This laryngeal sound is propagated along the trachea and the two main bronchi, 

 with gradually diminishing intensity (the bronchial sound). 



10. MEANS OF PROTECTION FOR THE LUNGS 



The air passages leading to the lungs (nasal cavities, throat, trachea and 

 bronchi) serve to protect the pulmonary alveoli from various kinds of injuries. 



The narrowness of the nasal passages and the bend which the air passage 

 makes in the pharynx serve the useful purpose of freeing the inspired air very 

 largely of its dust particles since the latter adhere to the mucus-covered walls 

 of one cavity or the other. This protection is largely wanting in breathing 

 by the mouth. The dust particles are also driven outward by the cilia of the 

 epithelium lining the air passages. This movement, especially in the parts 

 below the larynx, is of great service in keeping the alveoli free of dust. 



Of greater importance still is the fact that the parts of the respiratory 

 passages below the glottis under ordinary circumstances do not permit the 

 development of Bacteria: they are either entirely sterile or they contain an 

 insignificantly small number of Bacteria (Jundell). Since the tracheal 

 secretion possesses no antiseptic properties, this sterility must be accounted 

 for in some other way as yet quite unknown. 



Only in very exceptional cases does the inspired air have the temperature 

 of the body; but the expired air comes out warmed to the temperature of the 

 body and saturated with moisture. These changes occur chiefly in the wider 

 passageways so that the bronchi and especially the delicate alveoli are protected 

 from the harmful effects of loss of heat and loss of water. In fact, it has been 

 found that when, by means of an aspirator, air at 10-12 C. is taken in at- one 

 nostril and passed out at the other, entrance to the pharynx being closed, it 

 comes out warmed to 31 C. and saturated with moisture. If the outside tem- 

 perature be 0-4 C., it is warmed to 27.5 C. In similar experiments with 

 mouth breathing, the air reaching the pharynx was some 0.5 C. colder than 

 with nose breathing (Aschenbrandt, Kayser). From these observations we are 

 entirely justified in concluding that the air in the middle-sized bronchi at least 

 has acquired the temperature of the body, and is immediately saturated with 

 moisture at that temperature. 



The closure of the larynx which takes place in swallowing (page 281) 

 as well as different expiratory reflexes which are to be discussed in the fol- 

 lowing section are essentially for the protection of the lungs. 



SECOND SECTION 

 INNERVATION OF RESPIRATION 



1. THE EFFERENT NERVES 



Those muscles by the contraction of which the thoracic cavity is enlarged 

 or diminished in size (if we neglect the purely accessory muscles) receive 

 their nerves from the spinal cord : the nerves to the scaleni pass by way of the 



