320 



THE MUSCLES OF THE TRUNK. 



tendinous substance, and are directed obliquely downwards and forwards from the 

 prominent lower margin of one rib to the upper border of the next. The extent of 

 these muscles is generally from the tuberosities of the ribs to the outer ends of the 

 cartilages ; but in the upper three or four spaces they do not quite reach the ends of 

 the ribs, while in the last two intervals they are continued forwards along the lower 

 cartilages. Beyond the points at which the muscles cease the spaces are occupied 

 by the anterior intercostal aponeuroses, thin shining membranes, composed of fibres 

 having the same direction as those of the external intercostal muscles with which 

 they are continuous externally, and covering the fore parts of the internal inter- 

 costals. 



Variety. The supracostalis is a muscular slip occasionally seen, passing from the anterior 

 snd of the first rib downwards to the fourth, sometimes also to the second and third ribs. It 

 has been found connected above with the deep fascia of the neck, or with the scaleni muscles. 



Fig. 287. INTERCOSTAL MUS- 

 CLES OP THE FIFTH AND 

 SIXTH SPACES. (Allen 

 Thomson, after Cloquet. ) 

 k 



A, from the side ; B, from 

 behind. 



IV, fourth dorsal vertebra ; 

 V, V, fifth rib and cartilage ; 

 1, 1, levatores costarum mus- 

 cles, short and long ; 2, 2, 

 external intercostal muscle ; 

 3, 3, internal intercostal layer, 

 shown in the lower space by 

 the removal of the external 

 layer, and seen in A in the 

 upper space, in front of the 

 external layer : the deficiency 

 of the internal layer towards 

 the vertebral column is shown 

 in B. 



The internal inter- 

 costal muscles, thicker 

 in front than behind, 

 consist of fibres which 



incline downwards and backwards, but are somewhat shorter and less oblique 

 than those of the external muscles. Their attachments take place on the inner 

 surfaces of the ribs, the upper one being situated immediately above the sub- 

 costal groove, the lower one near the upper margin of the bone. Anteriorly 

 they reach to the sternal ends of the costal cartilages, and in the last two spaces 

 they become continuous with the internal oblique muscle of the abdomen : pos- 

 teriorly they extend as far as, or sometimes, especially in the upper spaces, slightly 

 beyond the angles of the ribs. Where the internal intercostals are deficient behind 

 the inner surface of each external intercostal is lined by the posterior intercostal 

 a/poneurosis, a membranous layer which is continuous internally with the anterior 

 superior costo-transverse ligament, and externally is prolonged into a thin fascia 

 between the two intercostal muscles. 



Delations. The internal are separated from the external intercostal muscles at the back 

 of the spaces by the intercostal vessels and nerves : they are lined internally by the pleura 

 and subpleural tissue (endotJioracic fascia}. 



The levatores costarum, twelve on each side, arise from the tips of the 

 transverse processes of the seventh cervical and the upper eleven dorsal vertebrse. 



