214 ACTION OF THE INDIVIDUAL RESPIRATORY MUSCLES. 



endeavor to escape laterally, and thus spread out themselves, as well as the adja- 

 cent thoracic wall. If the abdominal contents be removed from an animal, the 

 lower ribs are seen to be drawn inward with every contraction of the diaphragm ; 

 therefore, the presence of the viscera is necessary for the normal action of the 

 diaphragm. 



In order to obtain some idea as to the extent of thoracic enlargement due to 

 the action of the diaphragm, Landois carried out the following experiment: A 

 tracheal cannula was fastened in the body of a well-built, female, newly born 

 child that had died of hemorrhage. The body was completely immersed in water, 

 and the lungs were inflated. The vital capacity was estimated from the amount 

 of water displaced. The abdomen was then opened, the viscera removed, and a 

 wax cast was taken of the under surface of the diaphragm, with uninflated lungs 

 (that is, in the position of expiration). Hereupon, a quantity of air equal to the 

 determined vital capacity was introduced into the lungs, and after the air-passages 

 were closed, a second wax cast was taken of the diaphragm in this last position. 

 The difference in volume between these casts was determined, and it was found 

 that the proportion between the expansion due to the diaphragm and that due 

 to all other causes was 1:2^. These figures are, of course, only approximately 

 correct; for, in the first place, the removal of the abdominal viscera permits of 

 unimpeded descent of the diaphragm (which is, to a certain extent, compensated 

 for by the taking of the wax cast); and, secondly, the arch of the actively con- 

 tracting diaphragm presents a form differing from that produced passively by 

 inflation of the lungs. However, there is no other means at hand for determining 

 the thoracic expansion produced by the diaphragm. 



By increasing the intra-abdominal pressure, each diaphragmatic contraction 

 increases the flow of venous blood from the abdominal organs to the inferior 

 vena cava. 



The great importance of the diaphragm in the respiratory process can be 

 realized from the fact that bilateral section of the phrenic nerves in young rabbits 

 is followed by death. These nerves contain, as has been shown experimentally, 

 a few sensory fibers for the pleura, the pericardium, and a portion of the peri- 

 toneum. In animals, irritation of the lowest five intercostal nerves causes local, 

 inconsiderable contraction of the marginal part of the diaphragm. 



The contraction of the diaphragm, is not to be regarded as a simple muscular 

 contraction, for its duration is from four to eight times that of the latter. It is, 

 therefore, to be designated as a tetanic movement of short duration. 



2. The Elevators of the Ribs. At their vertebral extremity (which lies at a 

 much higher level than the sternal extremity) the ribs are articulated at their 

 heads and tubercles with the bodies and transverse processes of the vertebrae. 

 A horizontal axis passes through both joints, and upon this axis the rib is capable 

 of rotating upward and downward. If the axes of a pair of ribs be prolonged 

 from both sides until they meet in the middle line, an angle is formed that is 

 large (125) for the upper ribs and smaller (88) for the lower ones. An imaginary 

 plane may be passed through the arch of each rib, which inclines, in a state of 

 rest, from behind and inward, forward and outward. If the rib turns on its axis, 

 this inclined plane is raised more toward the horizontal. As the axes of the upper 

 ribs pass rather in a frontal direction and those of the lower ribs rather in a sagittal 

 direction, elevation of the upper ribs causes an expansion of the cavity from 

 behind forward, and elevation of the lower ones an enlargement from within 

 outward (as the movements of the ribs inclined downward are perpendicular to 

 their axes). At the same time the costal cartilages undergo slight torsion, which 

 brings their elasticity into play. 



All of the inspiratory muscles that act directly on the walls of the 

 thorax produce the desired result by elevating the ribs. In this con- 

 nection the following points are to be observed : (a) Elevation of the ribs 

 causes a widening of the intercostal spaces, (b) When the upper ribs 

 are elevated, all of the lower ribs and also the sternum are raised at the 

 same time, as all of the ribs are bound together by the soft structures in 

 the intercostal spaces, (c) During inspiration there occurs an eleva- 

 tion of the ribs and a widening of the intercostal spaces. An exception 

 is made of the lowest rib, which does not actually form a part of the 

 thorax. It moves downward, not upward, at least during deep in- 



