MUSCLES OF INSPIRATION. 119 



The orifice for the aorta is bounded by the bone and aponeurosis posteri- 

 orly, and in front, by a fibrous band to which the muscular fibres are attached, 

 so that their contraction has a tendency rather to increase than to diminish 

 the caliber of the vessel. 



The orifice for the vena cava is surrounded entirely by tendinous struct- 

 ure, and contraction of the diaphragm, although it might render the form of 

 the orifice more nearly circular, can have no effect upon its size. 



In ordinary respiration, the descent of the diaphragm and its approxima- 

 tion to a plane are the chief phenomena observed ; but as there is some re- 

 sistance to the depression of the central tendon, it is probable that there is 

 also a slight elevation of the inferior ribs. 



The phenomena referable to the abdomen which coincide with the de- 

 scent of the diaphragm can easily be observed in the human subject. As the 

 diaphragm is depressed, it necessarily pushes the viscera before it, and inspi- 

 ration is therefore accompanied by protrusion of the abdomen. This may 

 be rendered very marked by a forced or deep inspiration. 



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

 are limited chiefly to the cesophageal opening. The anatomy of the parts is 

 such that contraction of the muscular fibres has a tendency to close this 

 orifice. 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 impor- 

 tant 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 

 exclusively, by the phrenic nerve ; a nerve which, having the office of sup- 

 plying the most important respiratory muscle, derives its filaments from a 

 number of sources. It arises from the third and fourth cervical nerves, re- 

 ceiving a branch from the fifth and sometimes from the sixth. It then passes 

 through the chest, penetrates the diaphragm, and is distributed to its under 

 surface. Stimulation 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 ac- 

 tion 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 diaphragm, particularly hiccough and sobbing, which are produced by 

 spasmodic contractions of this muscle, generally not under the control of the 

 will. 



Action of the Muscles which elevate the Ribs. Scalene Muscles. In ordi- 

 nary 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 upon the first rib, must elevate with it, in inspiration, the 



