MUSCLES OF INSPIRATION. 125 



of the inferior ribs, the diaphragm assisting, in a limited degree it is true, the action 

 of the external intercostals. 



The phenomena referable to the abdomen, which coincide with the descent of the 

 diaphragm, can easily be observed in the human subject. As the diaphragm is depressed, 

 it necessarily pushes the viscera before it, and inspiration is therefore accompanied by 

 protrusion of the abdomen. This may be rendered very marked by a forced or deep 

 inspiration. 



The action of the diaphragm may be illustrated by a very simple yet striking experi- 

 ment. In an animal just killed, after opening the abdomen, if we take hold of the struct- 

 ures which are attached to the central tendon and make traction, we imitate, in a rough 

 way, the movements of the diaphragm in respiration, and the air will pass into the lungs, 

 sometimes with a distinctly-audible sound. 



The effects of the action of the diaphragm upon the size of its orifices are chiefly 

 limited to the cesophageal opening. The anatomy of the parts is such that contraction 

 of the muscular fibres has a tendency to close this orifice. When we come to treat of 

 the digestive system, we shall see that the contraction of the diaphragm is auxiliary 

 to the action of the muscular walls of the oesophagus itself, by which the cardiac open- 

 ing 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 almost exclusively, if not exclu- 

 sively, 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 some- 

 times from the sixth ; it passes through the chest, penetrates the diaphragm, and is dis- 

 tributed to its under surface. This nerve was the subject of numerous experiments by 

 the early physiologists, who were greatly interested in the minuti of the action of the 

 diaphragm and of other muscles, in respiration. Its galvanization produces convulsive con- 

 tractions of the diaphragm, and its section paralyzes the muscle almost completely. It 

 was noticed by Lower, that after section of both phrenic nerves the movements of the 

 abdomen were reversed, and it became retracted in inspiration. This is explained and 

 illustrated by voluntary suspension of the action of the diaphragm and exaggeration of the 

 costal movements. As the ribs are raised, the atmospheric pressure causes the diaphragm 

 to mount up into the cavity of the thorax, and of course the abdominal organs follow. 



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

 diaphragm and its constant and universal action in respiration, 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 dia- 

 phragm, particularly hiccough and sobbing, which are produced by spasmodic contrac- 

 tions of this muscle, generally beyond the control of the will. 



Action of the Muscles which elevate the Ribs. Scalene Muscles. In ordinary respira- 

 tion, 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 vertebra and the first 

 and second ribs. These muscles, which act particularly upon the first rib, must ele- 

 vate 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 very 

 short cartilage, which allows of very little movement, so that its elevation necessarily 

 carries with it the sternum. This movement increases both the transverse :md antero- 

 posterior diameters of the thorax, from the mode of articulation and direction of the 

 ribs, which are somewhat rotated as well as rendered more horizontal. 



Intercostal Muscles. Concerning the mechanism of the action of these muscle?, there 

 is great difference of opinion among physiologists; so much, indeed, that the author of 



