OF THE RESPIRATORY MUSCLES. - There are certain muscles especially destined to expand and contract the thoracic cavity, and there are others which act in different degrees as ac cessories, they may be classed as direct and indirect respiratory muscles.
The direct respiratory muscles are, inter costales extern and intend, levatores costarum, infra costales, triangularis stern, and the dia phragm. The indirect respiratory muscles are all those which straighten the spine or aid in fixing the whole body for the thoracic mus cles to act from as a fixed point, whilst by their other attachment they elevate or depress the ribs ; these are particularly the muscles of the neck and upper extremities, and those composing the walls of the abdo men. More indirectly still, the muscles of the limbs assist in respiration; —in difficult respiration, the patient seizes hold of any fixed object, whilst he employs his whole muscular force to assist in inspiration, or, as Boerhaave has expressed it, "scarcely any particle remains in the body which is not more or less concerned in the business of respiration."# The indirect respiratory mus cles, in fact, comprise nearly all the muscles of the body; therefore we shall only no tice the direct respiratory muscles. The di aphragm has already been described (art. DIAPHRAGM).
I. The intercostal muscles are arranged as two thin lamina; between the ribs ; one lamella is external to the other, hence they are named external and internal. The fibres of each layer are oblique in their direction in reference to the ribs, and each layer has its fibres disposed in a contrary direction to those of the other ; thus they are said to decussate. The twelve ribs form eleven intercostal spaces, conse quently there are eleven such decussating lamellm on each side of the thorax, and twenty two in all. Their attachments are to the in ferior border of one rib, and to the superior 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, i. 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 in their degree of obliquity relatively to those of other intercostal spaces Thus, broadly, it may layer, where it terminates with the osseous part of the rib, and fig. 672. the posterior view, commencing at the vertebra.
(b) Intercoatales 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 fig. 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 be stated that the external intercostal fibres crease in the degree of their obliquity as they proceed from the first to the last intercostal space; and that the internal intercostal fibres, on the contrary, decrease in the degree of their obliquity as they proceed in the same direc tion. Moreover, for the most part the ex ternal fibres increase in their obliquity in the same intercostal space as you proceed from the vertebrm towards the sternum, and the internal intercostal fibres, on the contrary, increase their obliquity from the sternum towards the vertebrm, therefore they seldom decussate at right angles to each other, or form a perfect cross like the letter X. This is their general relative position at death, but, during life, in every stage of respiration, their degree of obliquity varies. The obliquity of the intercostal fibres should be viewed more with reference to the spine than to the ribs, because we shall show that their action is relative to the spine, and not to the ribs and that they may be perpendicular to two ribs, while they are oblique to the spine, because the ribs are themselves oblique. We have never seen any of the external intercostal fibres perpendicular to a rib, but we may see that arrangement in the internal layer of the lower intercostal spaces (,fig. 673.). The omission of the relative posi tion of the spine with reference to the ob liquity of these muscles has led to many false conclusions as to their action in respiration. Let E E (fig. 675.) represent a spine or a rigid dicular to the two bars ; now move the bars up to p 3, also at an angle of 45° with E E, and the fibre L it becomes more oblique than at the position p 2. Therefore a tension may change from the oblique to the perpen dicular relatively to the ribs ; but it can never so change its relation to the spine. Thus L" it" and if it' between the bars at p 3 cross each other, in the same direc tion, but in different degrees of obliquity; when the bars are at p 4., they decussate in directions contrary with reference to the bars, but not with reference to the body E E. The position of the ribs is similar to the bars at i 4, therefore the decussation of the inter costal muscles must be viewed with reference to the spine. The intercostal fibres never cross each other like the lines L'it' and L"x" (fig. 675.), nor can they, by any change of movement, ever assume thatposition ; i. e. if they do not decussate in a direction con trary with relation to the spine, in no change of position, throughout the range of a semi circle, can they ever become directly decus sating fibres but when they directly cross each other as v D and VB (fig. 676.) relative to E E, in every other position to which the bars body, and A c, B u two levers representing ribs, allowing of free rotation at their centres of motion A and a. These two bars are per pendicular, or at 90°, with reference to the body E E; let L x represent a connecting tension or elastic fibre of any kind, this is oblique with reference to the two bars A c and s D, but move these bars down to the position of P 4, at an angle of 45° to the body E E, and the fibre L it becomes perpen-• can be moved, they will be seen still to main tain the same decussation.