THE MECHANICS OF BREATHING 285 



ment of the whole diaphragm, accompanied by a definite move- 

 ment of the abdominal viscera. 



The diaphragm may be regarded as consisting of two parts: (1) The 

 spinal, or crural, from the spinal column and arcuate ligaments to the 

 back portion of the central tendon; (2) the costo-sternal, or anterior, 

 attached to the front and sides of the central tendon, and arising by 

 several digitations from the ribs. The arch of the diaphragm rests 

 upon, and is supported equably by, the abdominal viscera, and is at 

 the same time kept constantly applied at the circumference to the 

 inner wall of the thorax by the negative intrathoracic pressure. Thus, 

 when its two parts contract, it acts like a true piston, moving in a 

 forward and downward direction. In quiet, normal breathing the 

 amount of movement of the right dome is about J inch, that of the 

 left dome and of the central tendon somewhat less. 



The Movement of the Ribs. In general, two movements of the ribs 

 are recognized as taking place in inspiration: (1) Round an axis cor- 

 responding to the spinal articulation, increasing the back-to-front 

 diameter of the thorax; (2) round one corresponding to the spino- 

 sternal articulation, increasing the diameter from side to side. Owing 

 to the variation in size, shape, inclination, and articulation of the ribs, 

 such an explanation, while essentially true in a general sense, is im- 

 perfect. It is better to divide the ribs into two sets: (1) The upper, 

 the second rib to the fifth; (2) the lower, the sixth rib to the tenth. 

 These two sets differ in their musculature, in the nature of their articu- 

 lation and ligaments, in their shape and arrangement, and in their 

 movements. 



It is better not to regard the first rib as one of the costal series, 

 but to associate it with the manubrium sterni, with which it performs 

 a special movement of its own. The lowest two ribs, inasmuch as 

 they are unattached in front " floating " are essentially parts of 

 the abdominal wall. Concerned in the respiratory movements, there- 

 fore, are 



1. The first rib and manubrium sterni. 



2. The upper costal series (2-5). 



3. The lower costal series (6-10). 



4. The floating ribs (11, 12). 



The First Rib and Manubrium Sterni. The first pair of ribs and 

 the manubrium sterni are intimately bound together, and form, 

 with the manubrium, a lid or operculum to the thorax. Behind this 

 lid is articulated with the spinal column, in front with the body of the 

 sternum, the manubrio-sternal joint (Fig. 152). During inspiration 

 there is a slight upward movement of the lid, allowed by the manubrio- 

 sternal joint, which causes an expansion of the anterior part of the 

 apex of the lungs. This movement is particularly marked in the 

 thoracic type of breathing. To demonstrate the movement at the 

 manubrio-sternal joint small mirrors are fixed above and below this 

 joint, and the movements of the reflected spots of light observed on 

 a- screen. During inspiration the spots diverge, during expiration 



