NEW VIEW OF THE MECHANISM OF RESPIRATION 291 



symmetrically disposed on the several regions of the trunk ; an arrangement which secures great accuracy of muscular 

 movement. Fig. 50 shows that the chest and abdomen form a closed, cylindrical, slightly compressed cavity : the 

 cyhnders being divided into two by the diaphragm. 



The arrangement of the muscles of the trunk is a repetition of that of the ventricles of the heart ; the only 

 difEerence being that in the heart there are no osseous attachments for the muscles, whereas in the trunk there are 

 many. This, however, is a matter of detail, and does not invahdate the important fact that the heart and chest 

 muscles are arranged on a common principle, and that they are invested with centripetal and centrifugal rhythmic 

 movements whereby they can close and open their several cavities in all their diameters. The movements are 

 spontaneous, inherent movements ; the closing movements not causing the opening movements, and vice versd. Both 

 sets of movements are vital in their nature. The cavity of the trunk, divided into two by the diaphragm, consti- 

 tutes what is practically an enormous heart ; the thoracic cavity representing the left auricle, the diaphragm the 

 bicuspid or mitral valve, and the abdominal cavity the left ventricle. It follows that the cardiac muscles supply 

 the pattern or type for the trunk muscles, and that the former pulsate within the latter. The cardiac muscles are 

 the precursors of the trunk muscles. 



The muscular arrangements which obtain in the trunk reappear in a well-marked form in the limbs, as is shown 

 further on. 



From the foregoing it will readily be inferred that the cardiac and trunk muscles have much in common, both 

 structurally and functionally, and that the trunk muscles are largely mixed muscles, that is, they are partly in- 

 voluntary and partly voluntary. The involuntary nature of the trunk muscles is generally overlooked, but, as a 

 matter of fact, the involuntary muscles are more numerous and important than the voluntary ones. 



Fig. 52 affords an admirable example of the typical muscular arrangements which everywhere obtain in the 

 body ; the muscles, as stated, running in vertical, shghtly obhque, oblique, and transverse directions. It also shows 

 that certain of the muscles (lower digitations of serratus magnus and attachments of external oblique) follow a 

 phcated and interdigitating arrangement, and that others (the latissimus dorsi or great muscle of the back) radiate 

 and spread out so as to assume a characteristic fan-shape. Lastly it shows that the powerful hip muscles (gluteus 

 maximus, medius, minimus, and others) of which the gluteus maximus and medius only are shown in the figure, run 

 in nearly every direction, as is the case in the shoulder muscles (scapular, deltoid, pectoralis muscles, &c.), and with 

 which they should be compared (see Figs. 51 and 53). 



The ball and socket or universal joints, as already indicated, can only be moved satisfactorily by muscles and 

 muscular fibres which run practically in every direction. Universal movements necessitate a universal arrangement 

 of muscles and muscular fibres. 



Fig. 52 further shows how certain of the movements of the thorax and abdomen are produced. The external 

 and internal obUque muscles and transverse abdominal ones, aided by the rectal abdominal muscles, when they 

 contract draw down the ribs and diminish the cavities of the thorax and abdomen in a direction from above down- 

 wards and so take a prominent part in the movements of expiration. When the thorax, pelvis, and diaphragm are 

 fixed, they powerfully compress the abdominal viscera, and so take a prominent part in defiecation, urination, and 

 parturition. The recti abdominal muscles are mainly muscles of expiration. They, however, in conjunction with 

 the transversahs abdominis muscles, which run at right angles to them, exercise a marked influence in diminishing 

 the cavity of the abdomen in its longitudinal and transverse diameters. If to the recti and transverse abdominal 

 muscles be added the obhque ones (external and internal oblique abdominal muscles), which as their names 

 indicate run in obUque directions and cross each other, the most powerful muscular arrangements conceivable for 

 strength and for efficiency in expelling air, fluids, semi-solids, and sohds are at once secured. It is the same arrange- 

 ment which obtains in the ventricles of the mammalian heart, the stomach, the bladder, and the uterus— viscera 

 expressly formed to receive, retain, and expel their contents at given intervals. In these viscera, as has been 

 already shown, the muscular fibres exercise a double power— a centripetal contracting power, and a dilatmg centri- 

 fugal power ; and I am disposed to beheve that all muscles, voluntary, involuntary, and mixed, exercise the double 

 power referred to. As the viscera in question cannot force the narrow passage of their sphincters, it follows that 

 the sphincters must dilate and open spontaneously when the bodies of the viscera close and contract— the sphincters 

 closing or contracting when the bodies of the viscera open and dilate. The centripetal and centrifugal muscular 

 movements are vital and not vito-mechanical in their nature. In the case of the heart, the auricles contract and 

 close when the ventricles relax and open, and vice versd. 



A similar arrangement is met with in the muscles which alternately increase and diminish the cavities of the 

 chest and abdomen. When the muscles of the abdomen contract and shorten, those of the thorax relax and 

 elongate. When, on the contrary, the muscles of the thorax contract and shorten, those of the abdomen relax and 

 elongate : the two sets of muscles display co-ordinated centripetal and centrifugal rhythmic movements. 



