THE THORAX. 369 



as being depressors of the ribs, hence muscles of expiration ; while 

 the function of the internal set he considers to be that of elevating 

 the ribs, and therefore regards them as muscles of inspiration. 



The form of the diaphragm, when its muscular tissue is re- 

 laxed, is that of a dome with its convexity upward. When the 

 muscular fibers contract they pull down the central tendon, and at 

 the same time become themselves less convex and straighter. This 

 movement constitutes the descent of the diaphragm, and results in 

 increasing the capacity of the thorax ; therefore the diaphragm is 

 a muscle of inspiration indeed, it is the most important of all the 

 inspiratory muscles. 



The abdominal cavity contains the abdominal organs liver, 

 stomach, spleen, intestines, etc. and in its descent the diaphragm 

 depresses these structures, which under the pressure yield to a cer- 

 tain extent, the protrusion of the abdominal walls aiding by 

 allowing this displacement to take place to a greater degree than 

 it would were they rigid. There is, however, a limit to the 

 amount that the central tendon can descend, and when this limit 

 is reached the tendon becomes a fixed point from which the mus- 

 cular tissue can act, and the effect is to raise the lower ribs to 

 which it is attached ; thus the capacity of the thorax is still more 

 increased. This descent amounts to from 5.5 mm. to 11.5 mm. in 

 quiet breathing and 42 during forced inspiration. Not only are 

 the abdominal organs depressed, but they are also compressed, and 

 their attachments put upon the stretch ; when, therefore, the mus- 

 cular tissue of the diaphragm ceases its contraction and begins to 

 relax, the elasticity of the depressed and compressed abdominal 

 contents and the abdominal walls tends to raise the diaphragm 

 into the position it occupied at the beginning of the respiratory 

 act. This ascent of the diaphragm is, therefore, a phenomenon 

 of expiration. 



Muscles of Forced Inspiration. Trapezius, latissimus dorsi, 

 rhomboideus minor, rhomboideus major, serratus posticus superior, 

 serratus posticus inferior, iliocostalis, quadratus lumborum, sterno- 

 mastoid, pectoralis major, pectoralis minor, subclavius. 



Trapezius. This muscle arises from the superior curved line 

 of the occipital bone, the ligamentum nuchse, spinous process of 

 seventh cervical, and the spinous processes of all the dorsal 

 vertebrae, and the supraspinous ligament, and is inserted into 

 the clavicle, acromion process, and spine of the scapula. 



Nerve-supply. The muscular branch of the spinal accessory 

 and branches from the anterior divisions of the third and fourth 

 cervical nerves. 



Latissimus Dorsi. It arises from the spinous processes of the 

 six lower dorsal and those of the lumbar and sacral vertebrae, the 

 supraspinous ligament, crest of the ilium, and 3 or 4 lower ribs, 

 and is inserted into the bicipital groove of the humerus. 



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