OF THE THORAX 423 



angular arrangement of the lower costal cartilages, the sternum is lifted upward and forward, 

 the ribs upward and slightly outward, and the diameters of the thorax are thus increased. 



The action of the Internal intercostals is in dhpute. Haller long ago taught that they act 

 together with the External intercostals as inspiratory muscles. Investigators have since endeav- 

 ored to show that they act as expiratory muscles. 1 Others believe that the Intercostal muscles 

 contract simultaneously and serve merely as strong septal supports which prevent the inter- 

 costal spaces from being pushed out or drawn in during respiration. Masoin and Du Bois Rey- 

 mond, 2 in experiments on animals, proved that the intercartilaginous portions of the Internal 

 intercostals contracted synchronously with the Diaphragm. 



The Diaphragm is the principal muscle of inspiration, and presents the form of a dome con- 

 cave toward the abdomen. The central part of the dome is tendinous, and the pericardium 

 is attached to its upper surface; the circumference is muscular. During inspiration the lowest 

 ribs are fixed, and from these and the crura the muscle fibres contract and draw downward 

 and forward the central tendon with the attached pericardium. In this movement the curva- 

 ture of the Diaphragm is scarcely altered, the dome moving downward nearly parallel to its 

 original position and pushing before it the abdominal viscera. The descent of the abdominal 

 viscera is permitted by the elasticity of the abdominal wall, but the limit of this is soon reached. 

 The central tendon applied to the abdominal viscera then becomes a fixed point for the action 

 of the Diaphragm, the effect of which is to elevate the lower ribs and through them to push forward 

 the sternum and the upper ribs. The right cupola of the Diaphragm, lying on the liver, has a 

 greater resistance to overcome than the left, which lies over the stomach, but to compensate 

 for this the right crus and the fibres of the right side generally are stronger than those of the 

 left. 



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

 expulsive effort. Thus, before sneezing, coughing, laughing, crying, or vomiting, and previous 

 to the expulsion of urine or feces, or of the foetus from the uterus, a deep inspiration takes place. 

 The height of the Diaphragm is constantly varying during respiration; it also varies with the 

 degree of distention of the stomach and intestines and with the size of the liver. After a forced 

 expiration the right cupola 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 cupola is a little lower 

 than the right. Halls Dally states that the absolute range of movement between deep inspiration 

 and deep expiration averages in the male and female 30 mm. on the right side and 28 mm. on 

 the left; in quiet respiration the average movement is 12.5 mm. on the right side and 12 mm. 

 on the left. 3 



Skiagraphy shows that the height of the Diaphragm in the thorax varies considerably with the 

 position of the body. It stands highest when the body is horizontal and the patient on his back, 

 and in this position it performs the largest respiratory excursions with normal breathing. When 

 the body is erect the dome of the Diaphragm falls, and its respiratory movements become less. 

 The dome falls still lower when the sitting posture is assumed, and in this position its respiratory 

 excursions are least in extent. These facts may, perhaps, explain why it is that patients suffering 

 from severe dyspnea are most comfortable and least short of breath when they sit up. When 

 the body is horizontal and the patient on his side, the two halves of the Diaphragm do not behave 

 alike. The uppermost half sinks to a level lower even than when the patient sits, and moves 

 little with respiration; the lower half rises higher in the thorax than it does when the patient 

 is supine, and its respiratory excursions are much increased. In unilateral disease of the pleura 

 or lungs analogous interference with the position or movement of the Diaphragm can generally 

 be observed skiagraphically. 



It appears that the position of the Diaphragm in the thorax depends upon three main factors, 

 viz.: (1) The elastic retraction of the lung tissue, tending to pull it upward; (2) the pressure 

 exerted on its under surface by the viscera; this naturally tends to be a negative pressure, 

 or a downward suction, when the patient sits or stands, and a positive or an upward pressure 

 when he lies; (3) the intra-abdominal tension due to the abdominal muscles. These are in a 

 state of contraction in the standing position and not in the sitting position ; hence, the Diaphragm 

 when the patient stands is pushed up higher than when he sits. 



The Levatores costarum being inserted near the fulcra of the ribs can exert but little action 

 on them; they act as rotators and lateral flexors of the vertebral column. 



The Triangularis sterni draws down the costal cartilages, and is therefore a muscle of expi- 

 ration. 



Mechanism of Respiration. The respiratory movements must be examined during (a) 

 quiet respiration, and during (6) forced respiration. 



1 Consult articles by Oleland in the Journal of Anatomy and Physiology, May, 1867, p. 209; Baumler, Obser- 

 vations on the Action of the Intercostal Muscles, Erlanger, 1860 (Ref. in New Syd. Soc.'s Year-Book for 1861, p. 

 69); Keen, Trans. Coll. of Phys., Phila., Third series, vol. i, 1875, p. 97; Flusser, Ueber die Wirkung der Mus- 

 culi Intercostales, Anat. Anz., April 16, 1908; Boecker, Anat. Anz., June 27, 1908. 



2 Zur Lehre von der Function der Musculi intercostales interni, Arehiv fur Physiologic, 1896, p. 85. 



3 Inquiry into the Physiological Mechanism of Respiration, Journal of Anatomy and Physiology, vol. xliii,. 

 1908. 



