812 SPECIAL PHYSIOLOGY. 



striped muscular fibres of the air-tubes is excited by electricity, as 

 well as by chemical and mechanical stimuli ; .and it has been suggested 

 that they may assist in expelling air from the lungs ; but the slow ac- 

 tion of organic muscular fibres, renders it unlikely that they co-oper- 

 ate in movements so rapid as those of respiration ; it is more probable 

 that they regulate the diameter of the air-tubes, and perhaps aid in 

 expelling mucus or other secretions from the smaller tubes. The cilia, 

 which exist throughout the air-tubes, from the entrance of the air-cells 

 upwards beyond the larynx, not only assist in the diffusion of moisture 

 over the interior of these tubes, but perhaps also in retaining particles 

 of dust which abound in the air, and so preventing their reaching the 

 air-cells, and likewise in impelling upwards towards the glottis, mucus, 

 and entangled particles of matter. The current produced by these 

 cilia, always sets in the upward direction. 



Ordinary expiration is undoubtedly aided by certain proper expi- 

 ratory muscles, especially by the internal intercostals. These muscles 

 occupy only the anterior three-fourths of the intercostal spaces, being 

 absent at the back part of the chest ; they are placed inside the ex- 

 ternal intercostals. Their fibres pass, in each space, from above, 

 downwards, and backwards, or in the opposite direction to the fibres 

 of the external intercostals. As already alluded to, the forepart of 

 these muscles near the sternum, especially of the four or five upper 

 ones, is said to assist in elevating the ribs in inspiration ; but, else- 

 where, these muscles depress the ribs, invert their lower edges, and 

 diminish the width of the intercostal spaces, thus acting as expiratory 

 muscles, diminishing the capacity of the thorax, both from before 

 backwards, and in width. Within the internal intercostals are situ- 

 ated the infracostah, small muscular bundles, having the same direc- 

 tion as the internal intercostals, but reaching over two or three spaces ; 

 they are also expiratory muscles. The triangulares sterni, small thin 

 muscles, found on the internal surface of the sternum and cartilages 

 of the true ribs, likewise co-operate in expiration ; some maintain, 

 however, that a portion of these muscles is inspiratory. The auxili- 

 ary muscles of expiration, are the upper part of the serratus magnus, 

 when the scapula is previously fixed ; the posterior inferior serrati, 

 whi.ch pass from certain dorsal and lumbar vertebrae upwards to the 

 last four ribs ; the quadrati lumborum muscles, which ascend from the 

 pelvis and lumbar vertebrae to the lower ribs ; certain portions of the 

 long muscles of the back, known as the erectores spinw ; and, lastly, 

 the abdominal muscles which are concerned in drawing the lower ribs 

 downwards and inwards, such as the external and internal ohliqui, the 

 tramversales, the recti, and pyramidales muscles. The abdominal 

 muscles assist, even in ordinary expiration, by supporting or raising 

 upwards and backwards the abdominal viscera, when the relaxation of 

 the diaphragm causes these to ascend. In extremely forcible expira- 

 tion, as in powerful inspiration, all the muscles of the body may be 

 brought into some action. It .is remarkable that a single small mus- 

 cle, the arytenoid (p. 207), which closes the aperture of the glottis, 

 may, by an act of the will, be made to counteract the powerful efforts 

 of the ordinary and auxiliary muscles of expiration. Under this 



