RUPTURE OF THE DIAPHRAGM. 235 



but this is far from being an infallible symptom of the disease. It accompanies 

 introsusception, as well as rupture of the diaphragm. The weight of the intes- 

 tines may possibly cause any protruded part of them to descend again into the 

 abdomen. 



This muscle, so important in its office, is plentifully supplied with blood- 

 vessels. As the posterior aorta passes beneath the crura of the diaphragm it 

 gives out sometimes a single vessel which soon bifurcates; sometimes two 

 branches, which speedily plunge into the appendices or crura, while numerous 

 small vessels, escaping from them, spread over the central tendinous expansion. 

 As the larger muscle of the diaphragm springs from the sides and the base of 

 the chest, it receives many ramifications from the internal pectoral, derived from 

 the anterior aorta ; but more from the posterior intercostals which spring from 

 the posterior aorta. 



The veins of the diaphragm belong exclusively to the posterior vena cava. 

 There are usually three on either side ; but they may be best referred to two 

 chief trunks which come from the circumference of the diaphragm, converge 

 towards the centre, and run into the posterior cava as it passes through the 

 tendinous expansion. 



The functional nerve of the diaphragm, or that from which it derives its prin- 

 cipal action, and which constitutes it a muscle of respiration, is the phrenic or 

 diaphragmatic. Although it does not proceed from that portion of the medulla 

 oblongata which gives rise to the glosso-pharyngeus and the par vagum, yet 

 there is sufficient to induce us to suspect that it arises from, and should be 

 referred to, the lateral column between the superior and inferior, the sensitive 

 and motor nerves, and which may be evidently traced from the pons varolii to 

 the very termination of the spinal chord. 



The diaphragm is the main agent in the work of respiration. The other 

 muscles are mere auxiliaries, little needed in ordinary breathing, but affording 

 the most important assistance, when the breathing is more than usually 

 hurried. The mechanism of respiration may be thus explained : — Let it be 

 supposed that the lungs are in a quiescent state. The act of expiration has 

 been performed, and all is still. From some cause enveloped in mystery — con- 

 nected with the will, but independent of it — some stimulus of an unexplained 

 and unknown kind — the phrenic nerve acts on the diaphragm, and that muscle 

 contracts ; and, by contracting, its convexity into the chest is diminished, and 

 the cavity of the chest is enlarged. At the same time, and by some consenta- 

 neous influence, the intercostal muscles act — with no great force, indeed, in 

 undisturbed breathing ; but, in proportion as they act, the ribs rotate on their 

 axes, their edges are thrown outward, and thus a twofold effect ensues: — 

 the posterior margin of the chest is expanded, the cavity is plainly en- 

 larged, and also, by the partial rotation of every rib, the cavity is still more 

 increased. 



By some other consentaneous influence, the spinal accessory nerve likewise 

 exerts its power, and the sterno-maxillaris muscle is stimulated by the anterior 

 division of it, and the motion of the head and neck corresponds with and assists 

 that of the chest ; while the posterior division of the accessory nerve, by its 

 anastamoses with the motor nerves of the levator humeri and the splenius, and 

 many other of the muscles of the neck and the shoulder, and by its direct influ- 

 ence on the rhomboideus, associates almost every muscle of the neck, the 

 shoulder, and the chest, in the expansion of the thorax. These latter are 

 muscles, which, in undisturbed respiration, the animal scarcely needs; but 

 which are necessary to him when the respiration is much disturbed, and to ob- 

 tain the aid of which he will, under pneumonia, obstinately stand until he falls 

 exhausted or to die. 



