INTERCOSTAL MUSCLES. 



237 



aponeurotic arch between them ; and externally, to an impression on the 

 ventral surface of the upper angle of the scapula. A middle part, which 

 is very thin, extends from the second, thii-d, and fourth ribs, to the base 

 of the shoulder bone. And a lower part, which is the strongest, is con- 

 nected on the one side with four ribs (fifth, sixth, seventh, and eighth), 

 where it digitates with like processes of origin of the external oblique 

 muscle ; and, on the other side, it is fixed into the special surface on the 

 costal aspect of the lower angle of the scapula. 



The serratus is applied against the ribs and the intercostal muscles, and 

 is partly concealed by the pectoral muscles and the axillary vessels and 

 nerves : in the ordinary position of the arm the scapula and subscapularis 

 are in contact with it. 



Action. The whole muscle acting, the scapula is carried forwards. But 

 the lower and stronger fibres can move forwards the lower angle, rotating 

 the bone around an axis through the centre, and raise the acromion. 



Dissection. The intercostal muscles will be brought into view by de- 

 taching the processes of origin of the serratus from the ribs for a couple 

 of inches, and by taking away the loose tissue on the surface. Towards 

 the front of the chest is a thin aponeurosis, which is continued forwards 

 from each external intercostal to the sternum ; this is to be retained in 

 the third intercostal space. Some of the lateral cutaneous nerves should 

 be preserved. 



The INTERCOSTAL MUSCLES are named from their position between the 

 ribs. There are two layers in each space, but neither occupies the whole 

 length of the space. The direction of the fibres differs in each stratum ; 

 for, whilst the fibres of the external 



muscle run very obliquely down- Fig- 71. 



wards and forwards, those of the 

 internal have an opposite direction 

 between the osseous parts of the 

 ribs, so that the two sets cross. 



The external muscle is fixed to 

 the outer margin of the ribs of each 

 intercostal space, and consists of 

 fleshy and tendinous fibres. Pos- 

 teriorly the fibres begin at or near 

 the tubercle of the rib ; and ante- 

 riorly they end short of the middle 

 line, but after a different manner in 

 the upper and lower spaces : In 

 the intervals between the true ribs, 

 they cease near the costal cartilages, 

 and a thin aponeurosis is continued 

 onwards from the point of ending to 

 the sternum. In the lower spaces 

 they are continued between the car- 

 tilages (.Theile) reaching the end 

 of the ribs in the last two. 



Dissection. The internal inter- 

 costal muscle will be seen by cutting through and removing the external 

 layer and the fascia in one of the widest spaces, say the second ; it will be 

 recognized by the difference in the direction of the fibres. 



Far back between the two muscles, and close to the rib above, the inter- 



DlAGRAM OP THE SERRATUS MAGNUS MUSCLE. 

 A. Attachments. 





w 

 I 



