400 RESPIRATION. [CHAP. XXIX. 



sternal ribs of birds, which are powerfully inspiratory, and slope 

 downwards and backwards from the sternum. These may have 

 their action explained by Hamberger's views, if the sternum be 

 regarded as giving the fulcrum instead of the vertebral column. 

 Again, the lowest external intercostal lengthens in inspiration, and 

 is an expiratory muscle, the eleventh rib in inspiration rising from 

 the twelfth, which is stationary.* 



On the whole we may conclude, that in inspiration, the upper 

 ribs rise by the action of the scaleni that the rest of the ribs in 

 their hinder part rise and open out by the action chiefly of the 

 external intercostals ; while in front the intercostal spaces are nar- 

 rowed above by the rise of the second, third, and fourth ribs to- 

 wards the first, and at the same time widened midway and below, 

 the flexibility of the costal cartilages having much to do with this 

 latter movement. It may, also, be regarded as certain that the 

 internal intercostals, except in front, are muscles of expiration, and 

 approximate the ribs. 



Dr. Sibson has well pointed out that the forces which expand 

 the thorax act, in a great degree, in a separate and independent 

 manner on its several parts, that the lower region, and the lower 

 lobes of the lungs which fill it, are enlarged by the diaphragm and 

 lower external intercostals, while the upper region expands under 

 the influence of the scaleni, its external intercostals, and (in 

 front) its internal intercostals. A variety of morbid conditions of 

 the lungs, where the expansibility of one or more lobes is modified, 

 illustrates this observation, for one side of the chest may be observed 

 to expand without the other, or a portion of one side without the 

 rest. How important this amount of independence of the parts of 

 the chest is to the preservation of life, under accident or disease, 

 hardly needs to be explained. 



* On February 26, 1851, Dr. Sibson exposed the.intercostal muscles in a dog 

 under chloroform, when we noted the following as facts : 1. The lower fibres 

 of the serratus magnus contract during inspiration ; the upper fibres lengthen. 

 2. The first five intercostal spaces diminish decreasingly during inspiration. 3. 

 The seventh, eighth, and ninth ribs diverge increasingly from the seventh to the 

 ninth, and the tenth, eleventh, and twelfth in a less degree. 4. The twelfth rib is 

 stationary. 5. The first external intercostal muscle shortens, the lowest 

 lengthens, during inspiration. 6. The first and second internal intercostal 

 muscles, in front, shorten during inspiration ; the third, also, in a very slight de- 

 gree. 7. The sixth internal intercostal elongates during inspiration, and the ninth 

 also, but in a greater degree. 8. The eleventh anterior external intercostal of 

 the diaphragmatic ribs shortens during inspiration. 9. Behind, the tenth ex- 

 ternal intercostal shortens, the tenth internal lengthens, during inspiration. 



