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 externi 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 intercostalcs 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 intercostals, 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 tebrm ; 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 urinates 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 costarunz (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 tebr?.
III. Triangularis sterni (Sterno-costa&, 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 digitations (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 rection of its fibres) the ribs. It may be said to be both inspiratory and expiratory.
IV. Infra-costales (Verheyen).—In connec tion with the inner surface of the ribs (fig. 673.) several small bundles of fleshy and tendinous fibres, which are thus named, will be found ex tending over two, and in some instances over three, intercostal spaces. They have the same direction with the internal intercostals, and are (properly) often described as parts of those muscles. The fasciculi vary in size and number, and may be found on any of the intercostal spaces, except, perhaps, the first ; but they are most constant on the lower ribs.
Action. — The same as the costal portion of the internal intercostals, depressors of the ribs and expiratory muscles.