4 THORAX 



serratus anterior and external oblique be careful not to injure the lateral 

 cutaneous nerves which make their appearance in the intervals between 

 their digitations. The anterior cutaneous nerves and perforating branches of 

 the internal mammary artery must also be preserved ; they pierce the 

 origin of the pectoralis major in the intervals between the costal cartilages, 

 close to the margin of the sternum. 



Intercostal Muscles and Membranes. These muscles and 

 membranes occupy the eleven intercostal spaces on each side. 

 In each space there are two strata of muscular fibres a 

 superficial and a deep. The superficial layer of muscular 

 fibres is called the external intercostal muscle, and the deep 

 layer the internal intercostal muscle. 



The external intercostal muscles are already exposed, and 

 very little cleaning is necessary to bring out their connections. 

 Note that entering into their constitution there is a large 

 admixture of tendinous fibres, and that these, as well as the 

 muscular fibres, are directed obliquely downwards and 

 anteriorly from the lower border of the rib above to the 

 upper border of the rib below. They do not extend farther 

 anteriorly, in the various spaces, than the region of union of 

 the bony with the cartilaginous parts of the costal arches. In 

 many cases, especially in the upper spaces, they do not 

 reach so far. When the muscular fibres stop, the tendinous 

 fibres are prolonged onwards to the sternum in the form 

 of a membrane, which is called the anterior intercostal mem- 

 brane. The external intercostal muscles of the two lower 

 spaces are exceptions to this rule. They extend anteriorly to 

 the extremities of the spaces. Posteriorly the muscles ex- 

 tend as far as the tubercles of the ribs, but this is a point 

 which can be satisfactorily demonstrated only after the thorax 

 has been opened. 



Dissection. To bring the internal intercostal imiscles into view it is 

 necessary to reflect the external intercostal muscles, and also the anterior 

 intercostal membranes. Divide them along the lower border of each 

 space, and throw them upwards. In effecting this dissection, care must be 

 taken of the intercostal vessels, which lie between the two muscular strata, 

 and of the lateral branches of the intercostal nerves. 



The internal intercostal muscles^ thus laid bare, will be seen 

 to be similar in their constitution to the external muscles. 

 The fibres, however, run in the opposite direction viz., from 

 above, obliquely downwards and posteriorly. Superiorly, each 

 is attached to the inner surface of the upper rib, immediately 

 above the costal groove ; inferiorly, it is attached upon the 



