THORAX. 



1043 



border of the next below. They do not ac- 

 company each other throughout the entire 

 intercostal space. These muscles, therefore, 

 differ from each other in two ways, in the 

 direction of their fibres, and in the extent of 

 their attachment, for neither set are prolonged 

 throughout the entire length of an intercostal 

 space. 



(a) Intercostales externi. These have their 

 fibres running obliquely downwards and for- 

 wards ; they are continued throughout 'the 

 whole osseous intercostal space, *. e. from the 

 tubercles of the ribs, to where the cartilages 

 commence ; here they terminate. Haller 

 once noticed these fibres " continued without 

 interruption to the sternum, filling up the in- 

 tercartilaginous spaces."* A thin aponeurosis 

 is prolonged from the free anterior margin of 

 this layer, up to the sternum. This muscular 

 layer is thicker than the internal layer. Fig. 

 671. represents the anterior extremity of this 



Fig. 671. 



Fig. 672. 



External intercostals. Anterior view. 



layer, where it terminates with the osseous 

 part of the rib, and^g. 672. the posterior view, 

 commencing at the vertebrae. 



(b) Intercostales interni. These, as their 

 name implies, are internal to the above layer. 

 Their fibres are likewise oblique, and have a 

 contrary direction, downwards and backwards 

 crossing the former layer. They commence 

 at the sternum, fill up the intercartilaginous 

 spaces, and part of the interosseous spaces, 

 and terminate at the angles of the ribs. 

 Fig. 671. represents them commencing at the 

 sternum, and disappearing behind the ex- 

 ternal layer. Figs. 673. and 674. show them for 

 the remainder of their course ; in^g. 673. they 

 will be seen to terminate short of the verte- 

 bral column. A thin aponeurosis is pro- 

 longed from their free margin backwards, to 

 the end of the intercostal spaces. 



All the intercostal fibres are oblique in their 

 direction, with reference to the spine and ster- 

 num. The fibres of one intercostal space differ 



* El. Phys. torn. iii. p. 29. 



External intercostals. Posterior view. 



in their degree of obliquity relatively to those of 

 other intercostal spaces Thus, broadly, it may 



Fig. 673. 



Internal intercostals. Posterior view, 



be stated that the external intercostal fibres in- 

 crease in the degree of their obliquity as they 



Fig. 674.. 



Internal intercostals. Anterior view. 



proceed from the first to the last intercostal 

 space j and that the internal intercostal fibres, 



3x2 



