RHYTHM OF RESPIRATION. 815 



monia ; a subcrepitant rhonchus arises from the bubbling of air through fluid 

 in the smaller air-tubes ; a sonorous rhonchus, with its snoring, rasping, and 

 cooing varieties, depends on obstructions in the larger air-tubes ; a sibilant, 

 or whistling, rhonchus is caused by mucous accumulations in the air-passages ; 

 and, lastly, a cavernous rhonchus is produced in cavities, or caverns, in the 

 lung, formed in the destructive stage of phthisis. Peculiar, harsh, rubbing, 

 or grating noises, named friction- sounds, frequently heard in inflammation of 

 the pleura, are caused by the rubbing of rough exudations of lymph eftused on 

 the pulmonary or costal pleura. Again, the relative amount of air and tissue 

 in different parts of the thorax, causes differences in the sounds produced by 

 percussion with the fingers, or otherwise, at different points of the thoracic sur- 

 face. The percussion-sound over the lungs is more hollow, or, as it is termed, 

 resonant, than over the heart, or great bloodvessels ; the ascent of the convex 

 liver into the concave base of the right lung, the two being of course separated by 

 the diaphragm, alters the percussion-sound, by diminishing the resonance at 

 the base of the right side of the chest. The chest is more resonant over the 

 great bronchi, than at other parts of the lungs. The resonance is everywhere 

 greater in thin persons. Various changes in the lungs produce great altera- 

 tions in the resonance of the corresponding parts of the chest ; congestions v 

 thickenings, consolidations, accumulations of fluid as in hydro- or heemo- 

 thorax, or empyema, and the presence of tumors, cause dulness in the percus- 

 sion-sounds ; whilst an abnormal amount of air, either in dilated or ruptured 

 air-cells, as in emphysema, or the cavities excavated in the lung-substance, as 

 in phthisis, or in the pleural chamber, as in pneumothorax, cause an increased 

 degree of resonance on percussion. Lasily, the term vocal resonance is applied 

 to the sound heard on the surface of the chest whilst the person is speaking ; 

 it is also modified by various internal conditions. A vibration felt on the 

 walls of the chest, during speaking, is called the vocal fremitus. 



The rhythmic movements of respiration are governed by a special 

 part of the nervous centres, co-operating with certain nerves. The 

 rhythmic movements of the heart, not yet fully explained, are performed 

 by a muscular organ, itself entirely uninfluenced by the will. But the 

 muscles of respiration are truly subject to the will. We can increase 

 or diminish the rapidity and force of these movements, according to 

 our pleasure ; we may imitate them, and can interrupt or arrest them 

 at any chosen point of the respiratory act. This latter power is, how- 

 ever, of limited duration. Very prolonged interruption of the respi- 

 ration produces convulsions. But, even after a period of from twenty 

 to thirty seconds, there arises, when the breath is held, a feeling so 

 distressing that it overcomes the most powerful volition. This feeling, 

 termed want of breath, at last irresistibly compels the resumption of 

 the respiratory acts. Like other sensations, it has its seat in some 

 portion of the gray substance of the nervous centres, and these being 

 irritated excite the motor nerves of the muscles of inspiration, which 

 are then thrown into involuntary action. The ordinary respiratory 

 movements are also involuntary ; xthey continue to be regularly per- 

 formed during the profouridest sleep, in a state of coma, in deformed 

 infants in which the cerebrum is deficient, and, for a time, even in 

 animals after the head has been removed. The respiratory movements 

 are, indeed, the most striking examples of reflex movements in the 

 body. Their nature has already been generally discussed . (p. 274). 

 The afferent or excitor nerves of inspiration are the pulmonary 

 branches of the pneumogastric and sympathetic nerves, which latter 

 contain fibres derived from the spinal cord ; also the cutaneous nerves 



