438 



DISSECTION OF THE THOKAX. 



but un- 

 equally. 



Thorax 



lesser 



how. 



Intercostal 

 muscles. 



Outer layer 

 is deficient 

 in front. 



Dissection 

 of deeper 

 muscle. 



Inner layer 



deficient 



behind. 



Use of 



outer 

 muscles ; 



in respiration ; for the muscle descends when air is taken into the 

 lungs, thus increasing the cavity ; and it ascends when the air is 

 expelled from those organs, so as to restore the previous size of the 

 space, or to diminish it in violent efforts. But the movement of the 

 diaphragm is not equal throughout, and some parts of the cavity 

 will be increased more than others. For instance, the central ten- 

 dinous piece, which is joined to the heart-case, moves but slightly ; 

 but the lateral, bulging parts descend freely, and increase greatly 

 the capacity of each half of the chest below by their separation 

 from the thoracic parietes. 



The thoracic cavity may be diminished by the diaphragm being 

 pushed upwards by enlargement, either temporary or permanent, of 

 the viscera in the abdomen ; or by the existence of fluid in the 

 latter cavity. 



Dissection. The external intercostal muscle should now be 

 carefully cleaned, care being taken to preserve the nerves and a thin 

 aponeurosis (anterior intercostal membrane) which passes forwards 

 from the muscle to the sternum at the front of the chest. 



The INTERCOSTAL MUSCLES form two layers in each space, but 

 neither occupies the whole length of the interval. The direction 

 of the fibres is different in the two, those of the external muscle 

 running very obliquely downwards and forwards, while those of the 

 internal pass, although less obliquely, downwards and backwards. 



The external muscle consists of fleshy and tendinous fibres, and is 

 attached to the margins of the ribs bounding the intercostal space. 

 It extends from the tubercle of the upper rib behind to the end of 

 the bone in front, except in the last two spaces, where the muscle 

 is continued forwards between the cartilages. The thin anterior 

 intercostal membrane takes the place of the muscle between the 

 rib-cartilages. 



Dissection. The internal intercostal muscle will be seen by 

 cutting through and removing the external intercostal and the mem- 

 brane in one of the widest spaces, say the third. 



The internal intercostal muscle passes from the inner surface of the 

 rib above to the upper border of the one below internal to the 

 attachment of the external intercostal muscle. It begins near 

 the angles of the ribs behind, the upper muscles approaching more 

 closely to the spine than the lower ones, and reaches to the 

 extremity of the intercostal space at the sternum in front. The fibres 

 of the lowest two muscles are continuous anteriorly with those of 

 the internal oblique of the abdomen. One surface is in contact 

 with the external muscle, and the intercostal vessels arid nerves ; 

 and the other is lined by the pleura. 



The hinder part of the muscles will be seen again in the dissection 

 of the back and thorax. 



Action. By the action of the intercostal muscles the ribs are 

 moved in respiration. 



The external intercostals elevate the ribs and evert the lower edges, 

 so as to enlarge the thorax in the aritero-posterior and transverse 

 directions : they come into play during inspiration. 



