IOTKA-COSTALES TKIANGULARI8 STERNI. 387" 



to diminish the capacity of the chest. If we bring an ani- 

 mal, a dog for example, completely under the influence of 

 ether, expose the walls of the chest, dissect off the fascia from 

 some of the external intercostals, then remove carefully a 

 portion of one or two of these muscles so as to expose the 

 fibres of the internal intercostals, it is not difficult, on close 

 examination, to observe the antagonism between the two sets 

 of muscles ; one being brought into action in inspiration and 

 the other in expiration. 



Infra-costales. These muscles, situated at the posterior 

 part of the thorax, are variable in size and number. They 

 are most common at the lower part of the chest. Their 

 fibres arise from the inner surface of one rib to be inserted 

 into the inner surface of the first, second, or third rib below. 

 The fibres follow the direction of the internal intercostals, 

 and acting from their lower attachments, their contractions 

 assist these muscles in drawing down the ribs. 



Triangularis Sterni. There has never been any doubt 

 concerning the expiratory function of the triangularis sterni. 

 From its origin, the ensiform cartilage, lower borders of the 

 sternum, arid lower three or four costal cartilages, it acts 

 upon the cartilages of the second, third, fourth, and fifth ribs, 

 to which it is attached, drawing them downwards, and thus 

 diminishing the capacity of the chest. 



The above-mentioned muscles are called into action in 

 ordinary tranquil respiration, and their sole function is to 

 diminish the capacity of the chest. In labored or difficult 

 expiration, and in the acts of blowing, phonation, etc., other 

 muscles, which are called auxiliaries, play a more or less 

 important part. These muscles all enter into the formation 

 of the walls of the abdomen, and their general action in 

 expiration is to press the abdominal viscera and diaphragm 

 into the thorax, and diminish its vertical diameter. Their 

 action is voluntary; and by an effort of the will it may be 



