340 RESPIRATION. 



tirely confined to the lower part of the chest. In labored respiration all 

 parts of the chest are alternately expanded and contracted, the breast rising 

 and falling as well in the male as in the female. We have now to consider 

 these several movements in greater detail, and to study the means by which 

 they are carried out. 



274. 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 diaphragmatic ; the latter causes the movement of the upper chest 

 characteristic 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 respiration by the 

 diaphragm is an important one ; in the male the diaphragm must be regarded 

 as the chief respiratory agent, and in some animals its use, for this purpose, 

 is so prominent that the movements of the ribs may in normal breathing be 

 almost neglected. In the rabbit for instance, 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 presents a convex surface 

 to the thorax ; when contracted it becomes much flatter, 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 

 downward and inward, and the lower false ribs upward and inward, toward 

 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. 



275. 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 upward from an oblique 

 to a more horizontal position, the sternal attachment must of necessity be 

 carried further away in front of the spine. Since all the ribs have a down- 

 ward slanting direction, they must all tend, when raised toward the hori- 

 zontal position, to thrust the sternum forward, some more than others, 

 according to their slope and length. The elasticity of the sternum and 

 costal cartilages, assisted by the articulation of the sternum to the clavicle 

 above, permits the front surface of the chest to be thus thrust forward as 

 well as upward, when the ribs are raised. By this action, the antero-pos- 

 terior diameter of the chest is enlarged. 



Since the ribs form arches which increase in their sweep as one pro- 

 ceeds from the first downward as far at least as the seventh, it is evident that 

 when a lower rib such as the fifth is elevated so as to occupy or to approach 



