THORAX. 



1055 



antagonise each other. In nature the external 

 intercostals are not continued over to the in- 

 tercartilaginous spaces. 



Fig. 700. 



In this model all the ribs are elevated with- 

 out the first bar being drawn up by any ten- 

 sion representing the scalenii muscles. In 

 fact the fulcra are the fixed points ; therefore 

 there are here six fixed points for the tensions 

 to act upon, and hence each row of oblique 

 tensions acts quite independently. 



This application of force and disposition of 

 the bars representing the ribs is after the 

 manner of that great engine of power the 

 "genou lever." (Vide Potter's Mechanics.) 



Tensions in the first space (fig. 700.) act 

 with a greater power in pushing out the body 

 b than do the tensions in the 5th space ; but, 

 on the other hand, the motion is less in the 

 1st than in the 5th space. It will also be 

 seen that these six bars, though moving all 

 equally, will produce an unequal effect upon 

 the body b b, forcing out the lower end more 

 than the upper end. The ribs of man, in the 

 same manner, increase in their length from 

 the 1st to the 8th, and, therefore, by an equal 

 mobility, an unequal protrusion of the sternum 

 is produced, advancing the lower end more 

 than the upper end. We have reason to believe 

 that the mobility of all the ribs is the same, 

 and that it is by their different lengths that the 

 different degrees of protrusion of the anterior 

 part of the thorax may be accounted for. 



Having investigated the effect of artificial 

 tensions acting on bars made to represent 

 the ribs, we can now return to the consider- 

 ation of the action of the direct respiratory 

 muscles. 



Action of the intercostal muscles (resumed). 

 The intercostales cxterni are all elevators of the 

 ribs. Inspiratory muscles. They separate the 

 ribs in the act, and they can do this indepen- 

 dently of any other muscle fixing the first rib. 

 The intercostales interni have a double action. 

 All those portions between the ribs depress 

 the ribs and are expiratory muscles; they also 

 approximate the ribs and in every way an- 

 tagonise the external intercostal muscles. 

 Those portions between the costal cartilages 



are elevators of the cartilages, and associates 

 with the external intercos.dls, and, thence, in- 

 spiratory muscles. These muscles also can 

 act independently of any other muscle fixing 

 the first or last rib. 



The intercostal muscles being antagonists, 

 they yield to each other, the same as any 

 other flexor yields to an extensor muscle. 

 We possess a perfect and definite command 

 over the ribs, and can stop their respiratory 

 movements at any stage of breathing, more 

 readily than we can those of the diaphragm. 



It is necessary to healthy breathing that 

 we should raise all the ribs ; they are there- 

 fore all mobile, and all their intercostal spaces 

 are increased in their perpendicular distance 

 during inspiration, and diminished during ex- 

 piration. 



II. Levatores costarum (Levatores breviores 

 costarum, Alb.). These are narrow, tendinous, 

 and fleshy fasciculi (fig. 672.) covering the pos- 

 terior end of the rib, and extending obliquely 

 downwards and forwards, in the same di- 

 rection as the external intercostals. Their 

 superior attachment is to the extremities of 

 the transverse processes of the dorsal ver- 

 tebrae ; their inferior to the margins of the 

 ribs between their angles and tubercles. 

 These fibres spread out and become flat at 

 their insertion. Each rib receives one from 

 the vertebra next above; there are therefore 

 twelve muscles on each side ; and that for 

 the first rib is derived from the last cervical 

 vertebra. The inferior muscles of this series 

 divide into two parts ; one of which is distri- 

 buted as above stated ; but the other, consist- 

 ing of longer fibres, passes over one rib and te-r 

 minates on the second below ; and thus each 

 of the lower ribs receives muscular fibres from 

 the transverse processes of two vertebrae. The 

 long bands have been described as separate 

 muscles under the name levatores longiores 

 costarum (Albinus). 



Action. The same as the external inter- 

 costals. Elevators of the ribs and inspi- 

 ratory muscles. Acting directly upon their 

 fulcra, the transverse processes of the ver- 

 tebrae. 



III. Triangularis sterni (Sterno-costalis, Ver- 

 heyen). A thin flat plane of muscular and 

 tendinous fibres (fig. 674.) placed within the 

 thorax, immediately behind the costal carti- 

 lages. This muscle is attached to the inner 

 surface of the ensiform cartilage, lower part of 

 the sternum, and the cartilages of the lower 

 true ribs. From these origins its fibres pass 

 laterally along the inner walls of the thorax, 

 diverging upwards, horizontally, and down- 

 wards, and are attached by dictations (which 

 give to the outer margin a serrated appear- 

 ance) into the cartilages, lower border, and 

 inner surface, of each of the true ribs, from the 

 5th to the 2nd inclusive. 



Action. This muscle varies considerably 

 in the extent and points of its attachment in 

 different bodies, and even in the opposite sides 

 of the same body. Hence, it aids in drawing 

 down and drawing up (according to the di- 



