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DESIGN IN NATURE 



trunk (thoracic and abdominal portions) as forming a large oval-shaped cylinder closed at both ends and divided into 

 two by a movable muscular partition — the so-called diaphragm to which allusion has already been made (Fig. 47, A, B). 



The muscles forming the cylinder are arranged in straight, curved, sUghtly obhque, oblique, very oblique, and 

 transverse lines ; bones being inserted among them to afford attachment and leverage. The respiratory movements 

 begin and terminate in the muscles, aided by the elasticity of the ribs and other structures. 



The straight muscles occur chiefly on the anterior and posterior aspects of the trunk ; the oblique, very oblique, 

 and transverse ones are found on all the aspects. 



Similar muscular arrangements are met with in the ventricles of the mammalian heart, in the face and neck, 

 in the pharynx and larynx, and in the anterior and posterior extremities. They are repetitions of the primary or 

 typical muscular arrangements which obtain in the hollow viscera. 



The straight muscles on the anterior aspect of the body are the rectus sternahs or sternahs brutorum (an occa- 

 sional muscle) (Quain, p. 243), the rectus abdominis, and the pyramidalis. The last two muscles draw down the 

 sternum, ribs, &c., in expiration. 



The straight muscles on the posterior aspect of the body are the quadratus lumborum and the dorsal muscles, 

 some of which are inserted into the ribs posteriorly. They fix the spinal column and posterior portions of the ribs 

 in forced respiratory movements. 



The obhque and very obhque muscles on the anterior aspect of the thorax are the pectoralis major and minor, 

 to which may be added the deltoid. These afford fixed points in difficult breathing. 



The oblique and very obhque muscles on the posterior aspect of the body are the trapezius, latissimus dorsi, supra 

 spinatus, infra spinatus, and rhomboidei, These also afford fixed points in difficult breathing. 



The oblique muscles on the lateral aspects of the thorax are the external and internal intercostals. These elevate 

 the ribs in inspiration. 



The oblique muscles on the lateral aspect of the abdomen are the external and internal oblique muscles. They 

 draw down the ribs in expiration. 



The transverse muscles in the thorax and abdomen are the serratus magnus, the serratus anticus and posticus, 

 and the transversalis abdominis. They contract and expand the thorax and abdomen as required. 



A combination of all these muscles, as far as the direction and arrangement of their muscular fibres are con- 

 cerned, is found in the diaphragm. In it, as explained, the muscular fibres run in straight lines, obliquely, very 

 obliquely, and transversely. 



Authorities are not agreed as to the number of muscles which take part in the respiratory movements either in 

 calm, normal respiration or in forced, violent respiration. 



As a matter of fact, nearly all the muscles of the body may take part in the respiratory movements if they be 

 very violent, as in asthma, severe dyspnoea, &c. In these cases the muscles of the neck, chest, abdomen, and even ' 

 the superior and inferior extremities are brought into play. The neck muscles connect the head with the upper 

 part of the thorax and afford fixed points : the muscles attached to the upper part of the humerus (when this is 

 elevated), as the shoulder, scapulae, and chest muscles, afford additional fixed points, and even the muscles of the lower 

 extremities attached to the pelvis perform a similar role. 



The number of muscles which take part in the respiratory movements is determined by the violence of the 

 respiratory acts, but even in calm or normal breathing the number is considerably in excess of those usually stated, 

 and I have to express my conviction that in every respiratory effort not only the muscles of the thorax, but also those 

 of the abdomen are engaged ; the thoracic and abdominal muscles, as indicated, being co-ordinated and acting 

 together ; the thoracic muscles shortening when the abdominal muscles elongate, and vice versd. The thoracic and 

 abdominal muscles act quite apart from elasticity as a moving force. 



This is a new explanation of the respiratory movements, but one to which I attach much importance, 

 as it brings the thoracic and abdominal nmscles into Une with the muscles of the mammahan heart, which 

 are typical. 



It will simphfy matters to regard all the muscles with fijced points above the ribs as muscles of inspiration, 

 and those with fixed points below the ribs as muscles of expiration. It may be stated in this connection that there 

 are two large muscles (the recti abdominis) to which hitherto no adequate function has been assigned. The recti 

 abdominis are doubtless muscles of expiration, and when they close or contract they pull the sternum, and the ribs 

 connected with it, downwards and inwards. This they do by a powerful direct pull. The external and internal 

 intercostal muscles are wholly concerned in elevating the ribs, which they do in conjunction with other muscles. By 

 their contraction they are largely instrumental in raising the ribs, and act as muscles of inspiration. The inter- 

 costal muscles do not act separately and in opposite directions, as is usually stated. They act simultaneously and 

 in the same direction. Their positions and connections, and the mode in which the ribs are jointed to the vertebral 



