314 RESPIRATION 



him to breathe through a tube opening to the exterior. The respiratory 

 movements are represented by the variations in the volume of the inclosed 

 air (Hering). 



B. MOVEMENTS OF THE RIBS 



The twelve ribs (Fig. 124) are thin, partly bony, partly cartilaginous hoops 

 projecting from each side of the thoracic vertebrae, and bending outward, for- 

 ward and downward, so as to inclose a space called the thoracic cavity. The 



FIG. 123. Respiratory curve of a rabbit. To be read from right to left. The downstroke 

 represents inspiration. The lower tracing is a time record in seconds. 



upper seven pairs are fastened in front directly to the sternum along the mid- 

 line of the body, while of the lower five pairs, two or three unite with the sternum 

 indirectly and the others end freely. 



Each rib is joined to its vertebra by two articulations, one with the centrum, 

 the other with the transverse process. Hence the axes around which the ribs 

 rotate in their movements are determined by the relative position of the two 

 articular surfaces. According to Landerer, the axes of the ribs from the first 

 to the tenth lie in horizontal planes, but are not parallel, the angles which the 

 axes make with the median plane being of unequal size. This angle for the first 

 rib is about 80, and decreases nearly uniformly from the first to the tenth 

 where it is 44. From this follows a fact very important for the study of the 

 movements of the ribs, and which has been confirmed also by direct observation, 

 namely, that the individual ribs by no means describe identical arcs. The axes 

 of rotation of the last two ribs are inclined at angles of 10 and 20 respectively 

 from the horizontal, while their intersecting angles with the median plane are 

 50 and 55 respectively. 



When the ribs are raised on their axes, in the first place the distance of 

 their anterior ends from the backbone is increased, and in the second place 

 the lateral parts of the ribs are carried outward. The thoracic cavity is en- 

 larged therefore in both the dorso-ventral and the transverse diameters. The 

 extent to which this enlargement takes place at the level of the individual 

 pairs of ribs depends upon the inclination of each and upon the intersecting 

 angle it makes with the median plane. The greater the inclination, the greater, 

 for ribs of equal length, becomes the dorso-ventral enlargement, and the smaller 

 the intersecting angle of the axis with the median plane, the greater is the 

 transverse enlargement. 



In this elevation and projection of ribs the sternum is of course advanced, 

 and this can only be accomplished by its rotation about a horizontal axis 

 passing through the upper end of the manubrium. Since, moreover, the dis- 



