OF THE THORAX. 355 



costartim assist the External intercostals in raising the ribs. The Triangularis 

 sterni draws down the costal cartilages ; it is therefore an expiratory muscle. 



The Diaphragm is the principal muscle of inspiration. When in a condition 

 of rest the muscle presents a domed surface, concave toward the abdomen ; and 

 consists of a circumferential muscular and a central tendinous part. When 

 the muscular fibres contract, they become less arched, or nearly straight, and 

 thus cause the central tendon to descend, and in consequence the level of the 

 chest-wall is lowered, the vertical diameter of the chest being proportionally 

 increased. In this descent the different parts of the tendon move unequally. 

 The left leaflet descends to the greatest extent ; the right to a less extent, on 

 account of the liver ; and the central leaflet the least, because of its connection to 

 the pericardium. In descending the diaphragm presses on the abdominal viscera, 

 and so to a certain extent causes a projection of the abdominal wall; but in conse- 

 quence Of these viscera not yielding completely, the central tendon becomes a fixed 

 point, and enables the circumferential muscular fibres to act from it, and so elevate 

 the lower ribs and expand the lower part of the thoracic cavity ; and Duchenne 

 has shown that the Diaphragm has the power of elevating the ribs, to which it 

 is attached, by its contraction, if the abdominal viscera are in situ, but that if these 

 organs are removed, this power is lost. When at the end of inspiration the Dia- 

 phragm relaxes, the thoracic walls return to their natural position in consequence 

 of their elastic reaction and of the elasticity and weight of the displaced viscera. 1 



In all expulsive acts the Diaphragm is called into action, to give additional 

 power to each expulsive effort. Thus, before sneezing, coughing, laughing, and 

 crying, before vomiting, previous to the expulsion of the urine and faeces, or of 

 the foetus from the womb, a deep inspiration takes place. 



The height of the Diaphragm is constantly varying during respiration, the 

 muscle being carried upward or downward from the average level ; its height also 

 varies according to the degree of distension of the stomach and intestines, and the 

 size of the liver. After a forced expiration, the right arch is on a level, in front, 

 with the fourth costal cartilage ; at the side, with the fifth, sixth, and seventh 

 ribs ; and behind, with the eighth rib, the left arch being usually from one to two 

 ribs' breadth below the level of the right one. In a forced inspiration, it descends 

 from one to two inches ; its slope would then be represented by a line drawn from 

 the ensiform cartilage toward the tenth rib. 



3/nscles of Jiix/tii-tift'on and Expiration. The muscles which assist the action 

 of the Diaphragm in ordinary tranquil inspiration are the Intercostals and the 

 Levatores costarum, as above stated, and the Scaleni. When the need for more 

 forcible action exists, the shoulders and the base of the scapula are fixed, and then 

 the powerful muscles of forced inspiration come into play; the chief of these are 

 the Trapezius, the Pectoralis minor, the Serratus posticus superior and inferior, 

 and the Rhomboidei. The lower fibres of the Serratus magnus may possibly assist 

 slightly in dilating the chest by raising and everting the ribs. The Sterno- 

 mustoid also, when the head is fixed, assists in forced inspiration by drawing up 

 the sternum and by fixing the clavicle, and thus affording a fixed point for the 

 action of the muscles of the chest. The Ilio-costalis and Quadratus lumborum 

 assist in forced inspiration by fixing the last rib (see page 367). 



The ordinary action of expiration is hardly effected by muscular force, but 

 results from a return of the walls of the thorax to a condition of rest, owing to 

 their own elasticity and to that of the lungs. Forced expiratory actions are 



Ph u*. No. II., May, 1SG7, p. 209, "On the Hutchinsonian Theory of the Action of the Intercostal 

 Muscles," who refers also to Henle, Luschka, Budge, and Baumler, Observations on the Action of the 

 Intercostal Muscles, Erlangen, 1 860. ( In New Syd. Soc.'s Year-Book for 1861 , p. 69. ) Dr. W. W. Keen has 

 come to the conclusion, from experiments made upon a criminal executed by hanging, that the Exter- 

 nal intercostals are muscles of expiration, as they pulled the ribs down, while the Internal intercostals 

 pulled the ribs up and are muscles of inspiration (Trans. Coll. Phys. Philadelphia, Third Series, vol. i., 

 187o. p. 97). 



1 For a detailed description of the general relations of the Diaphragm, and its action, refer to 

 Dr. Sibson's Medical Anatomy. 



