378 RESPIRATION. 



after a careful consideration of the relative position of the 

 parts. 



Levatores Costarum. The action of these muscles cannot 

 be mistaken. They have immovable points of origin, the 

 transverse processes of twelve vertebrae from the last cervical 

 to the eleventh dorsal, and, spreading out like a fan, are at- 

 tached to the upper edges of the ribs between the tubercles 

 and the angles. In inspiration they contract and assist in the 

 elevation of the ribs. They are more developed in man than 

 in the inferior animals. 



Auxiliary Muscles of Inspiration. The muscles which 

 have just been considered are competent to increase the ca- 

 pacity of the thorax sufficiently in ordinary respiration ; there 

 are certain muscles, however, which are attached to the ches* 

 and the upper part of the spinal column, or upper extremities, 

 which may. act in inspiration, though ordinarily the chest is 

 the fixed point, and they move the head, neck, or arms. 

 These muscles are brought into action when the movements 

 of respiration are exaggerated. When this exaggeration is 

 but slight and physiological, as after exercise, certain of them 

 (ordinary auxiliaries) act for a time, until the tranquillity of 

 the movements is restored. But when there is obstruction 

 in the respiratory passages, or when respiration is excessively 

 difficult from any cause, threatening suffocation, all the 

 muscles which can by any possibility raise the chest are 

 brought into action. The principal ones are put down in the 

 table under the head of extraordinary auxiliaries. Most of 

 these muscles can voluntarily be brought into play to raise 

 the chest, and the mechanism of their action can in this way 

 be demonstrated. 



Serratus Posticus Superior. This muscle arises from the 

 ligamentum nuchse, the spinous processes of the last cervical 

 and upper two or three dorsal vertebrae, its fibres passing 



