284 HUMAN PHYSIOLOGY 



dilate with each inspiration. This is due to the action of 

 the dilatores narium which contract synchronously with 

 the other muscles of inspiration. Again, if the larynx be 

 examined, it will be found that the vocal cords slightly 

 diverge from one another during inspiration. This is 

 brought about by the action of the posterior crico-arytenoid 

 muscles (p. 308). 



Forced Inspiration. This comparatively small group of 

 muscles is sufficient to carry out the ordinary act of inspira- 

 tion. But, in certain conditions, inspiration becomes forced. 

 A forced inspiration may be made voluntarily, often it is pro- 

 duced involuntarily. Such forced inspiration is well seen in 

 patients suffering from heart disease, in whom the blood is 

 not properly oxygenated, and by whom powerful efforts are 

 made to get as much air into the lungs as possible. In this 

 condition every muscle which can act upon the thorax to 

 expand it is brought into play. The body and spinal column 

 are fixed in the erect position. The head is thrown back and 

 fixed by the posterior spinal muscles. The arms and shoulders 

 are fixed usually by holding on to the sides or arms of the 

 chair and every muscle which can act from the fixed spine, 

 head and shoulder girdle upon the thorax is brought into 

 play. Normally, these act from the thorax upon the parts 

 into which they are inserted ; now they act from their in- 

 sertion upon their point of origin. The sterno-mastoids, 

 sterno-thyroids, and sterno-hyoids assist in elevating the 

 thorax. The serratus magnus, pectoralis minor, and upper 

 fibres of the pectoralis major, and the part of the latissimus 

 dorsi which passes from the humerus to the three last ribs, 

 also pull these structures upwards. The facial and laryngeal 

 movements also become exaggerated. 



B. In Expiration the various muscles of inspiration cease 

 to act, and the forces against which they contended again 

 contract the thorax in its three diameters. 



The elasticity of the lungs is no longer overcome by the 

 muscles of inspiration, and the external atmospheric pressure 

 acting along with it drives the chest wall inwards (see p. 279). 



The elasticity of the costal cartilages and in the erect 

 position the iveight of the chest wall cause the ribs again 

 to be depressed, and finally the elasticity of the abdominal 



