MUSCLES WHICH RAISE THE RIBS 99 



The effects of the action of the diaphragm on the size of its openings 

 are limited chiefly to the oesophageal opening. The anatomy of the 

 parts is such that contraction of the muscular fibres has a tendency to 

 constrict this opening. The contraction of the diaphragm is auxiliary 

 to the action of the muscular walls of the oesophagus itself, by which the 

 cardiac opening of the stomach is regularly closed during inspiration. 

 This may become important when the stomach is much distended ; for 

 descent of the diaphragm compresses all the abdominal organs and 

 might otherwise cause regurgitation of food. 



The contractions of the diaphragm are animated by the phrenic 

 nerve ; a nerve which, having the office of supplying the most important 

 respiratory muscle, derives its filaments from a number of sources. It 

 arises from the third and fourth cervical nerves, receiving a branch from 

 the fifth and sometimes from the sixth. It then passes through the chest, 

 penetrates the diaphragm and is distributed on its under surface. Stimu- 

 lation of this nerve produces convulsive contractions of the diaphragm, 

 and its section paralyzes the muscle almost completely. 



From the great increase in the capacity of the chest produced by the 

 action of the diaphragm and its constant and universal action in respira- 

 tion, it must be regarded as by far the most important and efficient of the 

 muscles of inspiration. 



Hiccough, sobbing, laughing and crying are due mainly to the action 

 of the diaphragm, particularly hiccough and sobbing, which are produced 

 by spasmodic contractions of this muscle, usually not under the control 

 of the will. 



Action of the Muscles which raise the Ribs. Scalene Muscles. In 

 ordinary respiration, the ribs and the entire chest are elevated by the 

 combined action of a number of muscles. The three scalene muscles are 

 attached to the cervical vertebrae and the first and second ribs. These 

 muscles, which act particularly on the first rib, must elevate with it, in 

 inspiration, the rest of the thorax. The articulation of the first rib with 

 the vertebral column is very movable, but it is joined to the sternum by 

 a short cartilage, which allows of very little movement, so that its eleva- 

 tion necessarily carries with it the sternum. This movement increases 

 both the transverse and antero-posterior diameters of the thorax, on ac- 

 count of the mode of articulation and direction of the ribs, which are 

 somewhat rotated as well as rendered more nearly horizontal. 



Intercostal Muscles. Concerning the mechanism of the action of these 

 muscles, there is considerable difference of opinion among physiologists ; 

 so much, indeed, that the question is still left in some uncertainty. The 

 most extended researches on this point are those of Beau and Maissiat 

 (1843), and of Sibson (1846). The latter seem to settle the question of 



