RESPIRATION. 575 



ascent of the ribs produces an increase in the antero-posterior diameter 

 of the thorax ; or, in other words, increases the distance between the 

 sternum and vertebral column. Therefore, this ascent of the ribs, so 

 far from diminishing the inspiratory effect of the diaphragm, tends even 

 to increase it. This may be rendered clear by the diagram (Fig. 247). 



At the same time the diaphragm contracts the abdominal organs are 

 pressed downward, and so cause a projection of the abdominal walls. 



In forced inspiration the power exerted on the ribs through the dia- 

 phragm is greater than that which tends to elevate the lower ribs. This 

 is especially the case when there is any obstruction to the entrance of 

 air into the lungs. In such cases there is a distinct constriction of the 

 thorax at the points of insertion of the fibres of the diaphragm. This 

 reduction in the circumference of the chest at these points is, however, 

 of but slight importance, since the increase of the thoracic cavity by the 

 greater descent of the diaphragm more than compensates for the decrease 

 in its circumference. It is probable that in all circumstances this de- 

 pression of the lower ribs would be more marked in strong contraction 

 of the diaphragm were it not for the fact that the descent of the ab- 

 dominal organs produces an increased tension in the abdominal walls, 

 and, therefore, offers a certain resistance to the production of this 

 constriction. 



Colin has estimated that in the horse the diaphragm in inspiration 

 descends from ten to twelve centimeters in the abdominal cavity, thus, 

 to this extent, increasing the long diameter of the thorax, while the 

 transverse diameter at the same time increases from three to four centi- 

 meters. 



The movements of the ribs in producing inspiration are much more 

 complicated. Each rib articulates by two facets with a costal cavity 

 formed by the junction of the ribs and two contiguous dorsal vertebrae. 

 The only movement possible in the ribs, therefore, must occur around a 

 line which passes between these two points of articulation ; or, in other 

 words, nearly coincides with the axis of the neck of the rib. If the ribs 

 were straight they would be only able to turn around on their own axes ; 

 since, however, the ribs are all curved in different degrees, the turning of 

 the rib around the axis of its neck causes every point of the rib to 

 describe an arc of a circle (Fig. 248). 



Further, the point of articulation of each rib with the vertebral 

 column is on a higher plane than its articulation with the sternum and 

 costal cartilages, the degree of inclination being greatest in the first rib, 

 least in the second, and then gradually increasing until the last rib forms 

 almost the same angle as the first. From this arrangement it is evident 

 that every elevation of the ribs will increase the distance between the 

 sternum and the vertebral column, while the rotation of the ribs will 



