INTERCOSTAL SERVES AND VESSELS. 439 



The internal intercostals act in a different way at the side and fore of inner 

 part of the chest. :les ' 



Between the osseous part of the ribs they depress and turn in interosseous 

 those bones, diminishing the size of the thorax ; and they are ^ ' 

 brought into use in expiration. 



Between the rib cartilages they raise the ribs, and are muscles intercartt* 

 of inspiration, like the outer layer. ^ nous 



Dissection. The intercostal vessels and nerves at the sides and 

 front of the chest are now to be examined. The intercostal arteries 

 which run from behind forwards are small and are not easily 

 dissected out except in a well-injected subject. The best guide to Expose 

 the intercostal nerve is the lateral cutaneous branch, and this should ^y^and 

 be gently pulled on and traced back to the parent trunk. The third vessels, 

 and fourth spaces may be devoted to the particular examination 

 of the nerves, and the fourth rib should be cut through at its junction 

 with its cartilage in front and as far back at the side as possible, 

 and the severed portion of rib shelled out from its internal peri- Remove a 

 osteum, great care being taken not to injure the subjacent pleura, ofrib n 

 The nerve and its accompanying vessej. should first be sought for 

 far back between the intercostal muscles close to the rib above. 



The INTERCOSTAL NERVES now seen only in the anterior half of intercostal 

 their extent, are the anterior primary branches of the dorsal nerves, nerves: 

 and supply the wall of the thorax. Placed at first between the course, 

 layers of the intercostal muscles below the corresponding artery, 

 each gives off the lateral cutaneous nerve of the thorax about mid- 

 way between the spine and the sternum. Then, much diminished termination, 

 in size, the nerve is continued onwards, at first in the substance of ar 

 the internal intercostal muscle, and afterwards between that muscle 

 and the pleura as far as the side of the sternum, where it ends as 

 the anterior cutaneous nerve of the thorax. Branches supply the branches, 

 intercostal muscles, and the triangularis sterni. 



The INTERCOSTAL ARTERIES which run from behind forwards intercostal 

 between the ribs are derived from the thoracic aorta in the case of bifScate : 

 the nine lower intercostal spaces (these being known as the aortic 

 intercostal arteries], and from the superior intercostal branch of the sub' 

 clavian in the case of the upper two spaces. They lie with the nerves 

 between the strata of intercostal muscles, and close to the upper 

 rib bounding the space. Near the angle of the rib the artery gives 

 off a collateral branch which is continued forwards along the edge 

 of the rib below ; and both it and the parent vessel anastomose in 

 front with the anterior intercostal offsets of the internal mammary 

 artery which run outwards. 



A small cutaneous offset is distributed with the lateral cutaneous offsets, 

 nerve of the thorax, and other branches are furnished to the 

 thoracic wall. 



Dissection. Make three saw cuts through the sternum, two Division of 

 transversely across, one opposite the middle of the first intercostal th 

 space, and the other between the junction of the sixth costal 

 cartilages, and with a third cut divide the piece of sternum between 

 the first two cuts longitudinally into two, taking care not to open 



