174 CHANGES IN THE CHEST. 



intra- abdominal pressure, favours the venous blood-current in the abdomen towards the vena 

 cava inferior. 

 Phrenic Nerve. The immense importance of the diaphragm as the great inspiratory muscle is 



S roved by the fact that, after both phrenic nerves (third and fourth cervical nerves) are divided, 

 eath occurs. The phrenic nerve contains some sensory fibres for the pleura, pericardium, and 

 a portion of the diaphragm. The contraction of the diaphragm is not to be regarded as a 

 " simple muscular contraction," since it lasts 4 to 8 times longer than a simple contraction; it 

 is rather a short tetanic contraction, which we n^ay arrest in any stage of its activity, without 

 bringing into action any antagonistic muscles {Kronecker and Marckwald). 



(2) The Elevators of the Bibs. The ribs at their vertebral ends (which lie much higher than 

 their sternal ends) are united by means of joints by their heads and tubercles to the bodies and 

 transverse processes of the vertebrae. A horizontal axis can be drawn through both joints, 

 around which the ribs can rotate upwards and downwards. If the axis of rotation of each pair 

 of ribs be prolonged on both sides until they meet in the middle line, the angles so formed are 

 greatest above (125), and smallest below (88). Owing to the ribs being curved, we can imagine 

 a plane which, in the passive (expiratory) condition of the chest, has a slope from behind and 

 inwards to the front and outwards. If the ribs move on their axis of rotation, this plane 

 becomes more horizontal, and the thoracic cavity is increased in its transverse diameter. As 

 the axis of rotation of the upper ribs runs in a more frontal, and that of the lower ribs in a more 

 sagittal direction, the elevation of the upper ribs causes a greater increase from before back- 

 wards, and the lower ribs from within outwards (as the movements of ribs which are directed 

 downwards are vertical to the axis). The costal cartilages undergo a slight tension at the same 

 time, which brings their elasticity into play. 



Changes in the Chest. All "inspiratory muscles" which act directly upon the 

 chest-wall do so by raising the ribs : (a) When the ribs are raised, the intercostal 

 spaces are widened, (b) When the upper ribs are raised, all the lower ribs and the 

 sternum must be elevated at the same time, because all the ribs are connected with 

 each other by means of the soft parts of the intercostal spaces, (c) During inspira- 

 tion, there is an elevation of the ribs and a dilatation of the intercostal spaces. 

 (The lowest rib is an exception : during forced respiration, at least, it is drawn 

 downwards.) (d) If, on a preparation of the chest, the ribs be raised as in inspira- 

 tion, we may regard all those muscles as elevators of the ribs, whose origin and 

 insertion become approximated. Every one is agreed that the scaleni and levatores 

 costarum longi et breves, the seri'atus posticus superior, are inspiratory muscles. 

 These are the most important inspiratory muscles which act upon the ribs. 



Intercostal Muscles. With regard to the action of the intercostal muscles, there 

 is a great difference of opinion. According to the above experiment, the external 

 intercostals and the intercartilaginous parts of the internal intercostals act as in- 

 spiratory muscles, whilst the remaining portions of the internal intercostals (as far as 

 they are covered by the external) are elongated when the ribs are raised, while they 

 shorten when the chest-wall descends. A muscle shortens only during its activity. 

 The internal intercostals were regarded by Hamberger as depressors of the ribs or 

 expiratory muscles. 



In fig. 139, I, when the rods, a and b (which represent the ribs), are raised, the intercostal 

 space must be widened (e/>c d). On the opposite side of the figure, it is evident that when 

 the rods are raised, the line, g h, is shortened (i k<g h, direction of the external intercostals) 

 I m is lengthened (I m<o n, direction of internal iutercostals). Fig. 139, II, shows, that when 

 the ribs are raised, the inter-cartilaginei, indicated by g h, and the external intercostals in- 

 dicated by I k, are shortened. When the ribs are raised, the position of the muscular fibres is 

 indicated by the diagonal of the rhomb becoming shorter. 



The mode of action of the intercostal muscles is an old story, Galen (131-203 a.d.) regarding 

 the externals as inspiratory, the internals as expiratory. Hamberger (1727) accepted this 

 proposition, and considered the intercartilaginei also as inspiratory. Haller looked upon both 

 the external and internal intercostals as inspiratory, while Vesalius (1540) regarded both as 

 expiratory. Landerer, observing that the upper two or three intercostal spaces became narrower 

 during inspiration, regarded both as active during inspiration and expiration. They keep one 

 rib attached to the other, so that their action is to transmit any strain put upon them to the 

 wall of the chest. On this view they will be in action, even when the distance between their 

 points of attachment becomes greater. Landois regards the external intercostals and inter- 

 cartilaginei as active only during inspiration, the internal intercostals only during expiration. 

 Martin and Hartwell exposed the internal intercostals, and observed whether they contracted 



