128 RESPIRATION. 



Lenator Anguli Scapula and Superior Portion of the Trapezius. Movements of the 

 scapula have often been observed in very labored respiration. Its elevation during in- 

 spiration is effected chiefly by the levator anguli scapulae and the upper portion of the 

 trapezius. The former muscle arises from the transverse processes of the upper three or 

 four cervical vertebrae and is inserted into the posterior border of the scapula below the 

 angle. It is a thick, flat muscle and, when the neck is the fixed point, assists in the ele- 

 vation of the thorax by raising the scapula. The trapezius is a broad, flat muscle, aris- 

 ing from the occipital protuberance, part of the superior curved line of the occipital 

 bone, the ligamentum nuchaa, and the spinous processes of the last cervical and all 

 the dorsal vertebras, to be inserted into the upper border of the spine of the scapula. 

 Acting from its attachments to the occiput, the ligamentum nuchae, the last cervical 

 vertebra, and perhaps one or two of the dorsal vertebrae, this muscle may elevate the 

 scapula and assist in inspiration. 



Pectoralis Minor and Inferior Portion of the Pectoralis Major. These muscles act 

 together to raise the ribs in difficult respiration. The pectoralis minor is the more effi- 

 cient. Tracing it from its attachment to the coracoid process of the scapula, its fibres 

 pass downward and forward to be attached by three indigitations to the external surface 

 and upper margins of the third, fourth, and fifth ribs just posterior to the costal cartilages. 

 With the coracoid process as the fixed point, this muscle is capable of powerfully assist- 

 ing in the elevation of the ribs. That portion of the pectoralis major which is attached 

 to the lower part of the sternum and costal cartilages is capable of acting from its in- 

 sertion into the bicipital groove of the humerus, when the shoulders are fixed, in concert 

 with the pectoralis minor. In great dyspnoea, it is frequently observed that the shoulders 

 are braced, the pectorals acting vigorously to raise the walls of the chest. 



Serratus Magnus. This is a broad, thin muscle covering a great portion of the lat- 

 eral walls of the thorax. Attached to the inner margin of the posterior border of the 

 scapula, its fibres pass forward and downward and are attached to the external surface 

 and upper borders of the eight superior ribs. Acting from the scapula, this muscle is 

 capable of assisting the pectorals in raising the ribs and becomes a powerful auxiliary in 

 difficult inspiration. 



We have thus considered the functions of the principal inspiratory muscles, without 

 taking up those which have an insignificant or undetermined action. In many animals, 

 the nares are considerably distended in inspiration; and, in the horse, which does 

 not respire by the mouth, these movements are as essential to life as the respiratory 

 movements of the larynx. In man, as a rule, the nares undergo no movement unless 

 respiration be somewhat exaggerated. In very difficult respiration, the mouth is opened 

 at each inspiratory act. We have not thought it necessary to treat of the action of those 

 muscles which serve to fix the head, neck, or shoulders in dyspnoea. 



The division into muscles of ordinary inspiration, ordinary auxiliaries, and extraor- 

 dinary auxiliaries, must not be taken as absolute. In the male, in ordinary respiration, 

 the diaphragm, intercostals, and levatores costarum are the great inspiratory muscles, 

 and the action of the scaleni, with the consequent elevation of the sternum, is commonly 

 very slight or may be wanting. In the female, the movements of the upper parts of 

 the chest are very marked, and the scaleni, the serratus posticus superior, and sometimes 

 the sterno-mastoid, are brought into action in ordinary respiration. In the various types 

 of respiration, the action of the muscles engaged in ordinary respiration necessarily pre- 

 sents considerable variations. 



Expiration. 



The air is expelled from the lungs, in ordinary expiration, by a simple and compara- 

 tively-passive process. The lungs contain a great number of elastic fibres surrounding 

 the air-cells and the smallest ramifications of the bronchial tubes, which give them great 

 elasticity. We can form an idea of the extent of elasticity of these organs, by simply 

 removing them from the chest, when they collapse and become many times smaller than 



