8 THORAX 



and abdomen so as to lay bare the costal arches and the external 

 intercostal muscles and membranes, but preserve the cutaneous 

 nerves and the accompanying vessels. Clean the external inter- 

 costal muscles from behind forwards and note that in the upper 

 spaces they are not prolonged between the costal cartilages, 

 but terminate at or near the sternal ends of the bony parts of 

 the ribs ; the intervals between the muscles and the sternum 

 are occupied by membranes, the anterior intercostal mem- 

 branes, 1 which cover the anterior parts of the internal intercostal 

 muscles. 



Musculi et Membranse Intercostales. The intercostal 

 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 is called the internal intercostal muscle. 



Musculi Intercostales Externi. Numerous tendinous fibres 

 are intermingled with the muscle fibres of the external inter- 

 costal muscles, and both the muscle fibres and the tendinous 

 fibres are directed obliquely downwards and forwards from 

 the lower border of the rib above to the upper border of the 

 rib below. The muscles do not extend farther forwards, in 

 the various spaces, than the region of union of the bony parts 

 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 pro- 

 longed onwards to the sternum in the form of a membrane 

 which is called the anterior intercostal membrane. The external 

 intercostal muscles of the lower two spaces are exceptions to 

 this rule. They extend forwards to the extremities of the 

 spaces. Posteriorly, the muscles extend as far as the tubercles 

 of the ribs, but that is a point which cannot be satisfactorily 

 demonstrated at the present stage of dissection. 



Dissection. To bring the internal intercostal muscles into 

 view it is necessary to reflect the external intercostal muscles, 

 and also the anterior intercostal membranes. Divide two or 

 more of the muscles and membranes along the lower borders of 

 the spaces in which they lie, and throw each muscle and mem- 

 brane upwards, but avoid injury to the intercostal vessels, which 

 lie in the space between the external and internal intercostal 

 muscles, and to the lateral branches of the intercostal nerve of 

 the space. 



1 The anterior intercostal membranes are sometimes called anterior inter- 

 costal ligaments, but such terminology is not justified either by their position 

 or constitution. 



