DEEP INSPIRATION. 809 



In deep inspiration, many more muscles come into play than in 

 ordinary breathing. Thus, the anterior and posterior scaleni muscles, 

 which descend, on each side, from the cervical vertebrae to the first 

 and second ribs, aid powerfully in elevating those ribs, and, through 

 them, perhaps all the others. The posterior superior serrati muscles, 

 situated deeply, one on each side of the back of the chest, must also 

 raise certain of the ribs. The cervicales ascendentes muscles will have 

 a similar action. If the scapula, or shoulder bone, and the clavicle, 

 or collar bone, are previously fixed by the muscles which descend to 

 them from the head and neck, viz., by the trapezius (Fig. 5, 2), sterno- 

 mastoid, levator of the angle of the scapula, and greater and lesser 

 rhomboid muscles on either side, then a very large and important mus- 

 cle, the great serratus (Fig. 4, 5), which passes from the base of each 

 scapula, over the sides of the chest, to the eight upper ribs, must also 

 assist powerfully in expanding the chest, by raising and drawing the 

 ribs outwards. So, likewise, in front of the thorax, the mbclavius 

 muscle, which passes from the collar-bone to the first rib, and the 

 lesser pectoral muscle, which descends from the scapula to the third, 

 fourth, and fifth ribs, on each side, will then serve to elevate the ribs. 

 In still more forced inspiration, when even the arms are fixed, by 

 holding on to some external object of support, the great pectoral mus- 

 cles in front (Fig. 4, 2), and the latissimi dor si behind (Fig. 5, 3), both 

 of which, besides other attachments, are connected, on the one hand, 

 with the humerus, and, on the other, with certain of the ribs, may then 

 co-operate in the elevation and outward rotation of these latter bones, 

 and thus assist in inspiration. All the muscles just mentioned, are 

 named auxiliary muscles of inspiration ; but a few of them only, those 

 first described, are ordinarily employed in deep inspiration. In very 

 extreme cases, however, nearly every muscle of the body may assist 

 in inspiration, by fixing certain parts, and thus affording more efficient 

 points of action to the proper respiratory muscles. 



The chief work performed in the act of inspiration, consist in over- 

 coming the elastic resistance of the costal cartilages, and lifting the 

 weight of the ribs. The lungs themselves are passive, or rather their 

 elasticity has to be overcome. These organs, becoming inflated in 

 every direction from their roots, as they follow the thoracic walls, are 

 necessarily enlarged in all directions, antero-posteriorly, transversely, 

 and vertically. The elastic fibres of the air-tubes yield both in a 

 longitudinal and a circular direction. The elastic walls of the air- 

 cells are extended generally; so also are the interlobular and sub- 

 serous areolar tissue, and the pulmonary and costal pleurae. To 

 facilitate the ingress of air, the air-passages, from the nose and mouth 

 down to the interior of the lungs, are supported either by bones or 

 cartilages, as, for example, by the cartilaginous alae of the nostrils, the 

 bones of the nasal cavities, the cartilages of the larynx, the incomplete 

 cartilaginous hoops of the trachea arid primary bronchi, and lastly, 

 by the less regular, but well-adapted plates of cartilage of the sec- 

 ondary bronchi and the bronchia. As especially fitted to maintain 

 the perviousness of the bronchia, the cartilaginous spurs placed at 

 the angles of bifurcation of those tubes, deserve particular mention. 



