CHAP. IL] RESPIRATION. 573 



331. Inspiration. There are two chief means by which the 

 chest is enlarged in normal inspiration, viz. the descent of the 

 diaphragm and the elevation of the ribs. The former causes that 

 movement in the lower part of the chest and abdomen so 

 characteristic of male breathing, which is hence called diaphragm- 

 atic ; the latter causes the movement of the upper chest character- 

 istic of female breathing, which is called costal. These two main 

 factors are assisted by less important and subsidiary events. 



Even in the female human subject, the share taken in respira- 

 tion by the diaphragm is an important one, in the male it is still 

 more important, and in some animals the use of the diaphragm for 

 this purpose is so prominent that the movements of the ribs may 

 in normal breathing be almost neglected. In the rabbit for in- 

 stance, in normal breathing, almost all the respiratory work is done 

 by the contractions of the diaphragm. 



The descent of the diaphragm is effected by means of the 

 contraction of its muscular fibres. When at rest the diaphragm, 

 drawn up by the negative intrathoracic pressure, presents a convex 

 surface to the thorax; when contracted it becomes much natter, 

 and in consequence the level of the chest-floor is lowered, the 

 vertical diameter of the chest being proportionately enlarged. In 

 descending, the diaphragm presses on the abdominal viscera, and 

 so causes a projection of the flaccid abdominal walls. From its 

 attachments to the sternum and the false ribs, the diaphragm, 

 while contracting, naturally tends to pull the sternum and the 

 upper false ribs downwards and inwards, and the lower false ribs 

 upwards and inwards, towards the lumbar spine. In normal 

 breathing, this tendency produces little effect, being counteracted 

 by the accompanying general costal elevation, and by certain 

 special muscles to be mentioned presently. In forced inspiration, 

 however, and especially where there is any obstruction to the 

 entrance of air into the lungs, the lower ribs may be so much 

 drawn in by the contraction of the diaphragm, that the girth of 

 the trunk at this point is obviously diminished. 



332. The elevation of the ribs is a much more complex 

 matter than the descent of the diaphragm. If we examine any 

 one rib, such as the fifth, we find that while it moves freely on its 

 vertebral articulation, it inclines when in the position of rest in 

 an oblique direction from the spine to the sternum ; hence it is 

 obvious that when the rib is raised, its sternal attachment must 

 not only be carried upward, but also thrown forward. The rib 

 may in fact be regarded as a radius, moving on the vertebral 

 articulation as a centre, and causing the sternal attachment to 

 describe an arc of a circle in the vertical plane of the body ; as the 

 rib is carried upwards from an oblique to a more horizontal position, 

 the sternal attachment must of necessity be carried farther away 

 in front of the spine. Since all the ribs have a downward slanting 

 direction, they must all tend, when raised towards the horizontal 



