292 DESIGN IN NATURE 



The dome-shaped diaphragm possesses the same powers. When it contracts and flattens, it enlarges the thorax 

 at the expense of the abdomen ; the cavity of the thorax being greatest when that of the abdomen is least, and the 

 converse. The obvious conclusion is, that the muscles of the thorax and abdomen and the diaphragm or muscular 

 partition between them have the power of increasing and diminishing the cavities of the chest and abdomen 

 spontaneously and independently. 



When the muscles of the thorax and diaphragm contract and shorten they enlarge the cavity of the chest in 

 all its diameters as in inspiration. When, on the other hand, the muscles of the diaphragm relax and those of 

 the abdomen contract they diminish the cavity of the chest in all its diameters as in expiration. The muscles 

 of the abdomen by their contractions assist in diminishing the cavity of the thorax by pulhng down the ribs 

 and pushing up such viscera as are placed beneath the diaphragm. The thoracic and abdominal muscular 

 arrangements are necessary to each other in the same way that the auricular and ventricular arrangements of the 

 heart are. 



While I venture to claim spontaneous, independent, rhythmic movements for the thoracic, diaphragmatic, and 

 abdominal muscles, I am not unmindful of the fact that the nerves with which these muscles are supplied exercise 

 a certain influence, and that if the ganglion of the tuber annulare of the brain be destroyed the power to breathe 

 is suddenly arrested. This does not prove that the nervous system is to be accredited with the production and 

 maintenance of the rhythmic movements of the chest, diaphragm, and abdomen. The nervous system at best 

 exercises a regulating power. In the case of the heart, the rhythmic movements of the auricles and ventricles con- 

 tinue for some considerable time after the heart is removed from the body and the connection between it and the 

 brain severed. Of course it may be said that the heart contains on its blood-vessels, on its surface, and in its sub- 

 stance large numbers of nerve ganglia, but this only goes to show that the heart, thorax, and abdomen can exercise 

 independent and co-ordinated rhythmic movements as apart from the brain ; the movements in question being, 

 for the wisest of purposes, placed beyond our control. It could not well be otherwise. The circulation, the respira- 

 tion, and the intake, digestion, and output of food and excreta generally are absolutely necessary to Ufe, and, if 

 the several processes had to be superintended by individuals, it goes without saying that hfe would be a burden 

 and in many ways undesirable. 



The relations which obtain between the cavities of the heart and those of the thorax and abdomen are of the 

 closest kind, and the fact that the muscles of the heart act without bones, while those of the thorax and abdomen 

 act in conjunction with bones, makes no difference, for, as I have already shown, the osseous system is, in every case, 

 a mere auxihary or adjunct of the muscular system. Movement always begins and terminates in muscle or hving 

 sarcode of some kind. The bones cannot move of themselves. 



Grave discrepancies occur in the descriptions given of the action of the respiratory muscles. 



While authorities are not agreed as to the number of muscles which take part in respiration, neither are they 

 agreed as to the functions performed by individual muscles and as regards their so-called active and passive move- 

 ments. This is especially true of the intercostal muscles. According to Cruveilhier these muscles neither act as 

 elevators nor depressors of the ribs, but only as tensors of the intercostal spaces. 



According to Halle the intercostals act together and are elevators of the ribs. 



Accordmg to Hamberger the external intercostals act as elevatora of the ribs — the internal intercostals as 

 depressors. 



According to Hutchinson the external intercostals and the parts of the internal intercostals placed between 

 the costal cartilages elevate the ribs, whereas the lateral portions of the internal intercostals depress the ribs. 

 Hutchinson's view is that commonly accepted at present. 



It has been sought to illustrate Hutchinson's theory by the aid of a model consisting of two straight parallel 

 bars of wood representing the ribs, a vertical upright representing the vertebral column, and two oblique elastic 

 bands representing the external and internal intercostal muscles. The parallel bars are jointed at one end to the 

 vertical upright, and the free ends of the bars are furnished with a piece of wood to represent the sternum. When 

 the elastic band which slopes downwards and forwards and represents the external intercostals is put upon the 

 stretch and allowed to recoil it elevates the bars. When the band which slopes downwards and backwards and 

 represents the internal intercostals is similarly treated it depresses the bars. 



This ingenious illustration, at first sight very convincing, is, upon careful examination, altogether unsatis- 

 factory, from the fact that the conditions are not identical. The ribs are not straight and parallel structures. 

 On the contrary, they are spiral and variously curved, are set at different angles to the spinal column, are 

 of different sizes, and during their rise and fall rotate upon their axes and within the sockets formed for them 

 by the ribs. Neither do the oblique elastic bands of the model aptly or accurately represent the action of the 

 intercostal muscles. 



