534 EESPIRATION 



Keeping these facts clearly in mind, it is now possible to assign a 

 definite cause to the taking of the first breath. In utero, the respira- 

 tory center of the fetus is not subjected to a stimulation by the carbon 

 dioxid, because its blood and tissues are constantly kept in an apneic 

 condition. Subsequent to the obliteration of the umbilical blood- 

 vessels, however, the carbon dioxid accumulates very rapidly and 

 finally incites the center to send out those impulses which give rise to 

 the first respiratory movement. This process is materially hastened 

 by mechanical and thermal stimuli, because the conditions which the 

 fetus meets with during and directly after the period of labor are 

 very different from those to which it has been subjected in utero. It 

 exchanges a practically indifferent medium heated to the temperature 

 of the body, with one much cooler and teeming with mechanical im- 

 pacts of all sorts. 



The Innervation of the Upper Respiratory Passage. — With the 

 exception of a small patch of modified epithelium forming the so-called 

 olfactory area, all sensory impressions from the mucous membrane of 

 the nose are relegated to the system of the trigeminal nerve. In 

 accordance with the character of the stimulus, these afferent impulses 

 give rise either to an acceleration or a retardation of the respiratory 

 movements. In the latter case, respiration may be arrested with the 

 chest in either the inspiratory or expiratory position. It need scarcely 

 be mentioned that the impulses generated in the nasal cavity, are first 

 relayed to the respiratory center by way of the trigeminus and are 

 then conveyed to the different muscles of respiration. These stimuli 

 are usually followed by an active expiration, the blast of air being ex- 

 pelled through the nasal cavity, while the oral cavity is temporarily 

 shut off by the closure of the fauces. This constitutes the act of 

 sneezing, the purpose of which is to dislodge the irritating body from 

 the nose. 



A similar reflex mechanism for safeguarding the respiratory passage 

 is situated in the pharynx. The lining of this cavity is innervated 

 in a sensory way by the glossopharyngeal nerves. Moderate excita- 

 tions occurring in the reahn of these nerves are immediately followed 

 by an inhibition of respiration and an active expiration, but in this 

 case the posterior nares are closed and the expiratory blast of air is 

 expelled through the oral cavity. This constitutes the act of coughing. 

 These impulses from the terminals of the glossopharyngeus are of 

 special value during the act of swallowing, because they lead to a 

 temporary arrest of the inspiratory movement and a closure of the 

 epiglottis so that the food cannot be aspirated into the laryngeal cavity. 

 The path pursued by these impulses is the same as that outlined pre- 

 viously, i.e,, they are first relayed to the nucleus of this nerve and to 

 the respiratory center, whence they are directed to the muscles of 

 respiration. 



On passing into the cavity of the larynx another nerve is met with, 

 namely, the superior laryngeal branch of the vagus (Fig. 263). It is a 



