

THE MUSCLES OF THE THORAX 407 



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 body of 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 fibers 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 fetus from the uterus, a deep inspiration takes place. 

 The height 1 of the diaphragm is constantly varying during respiration; it also varies with the 

 degree of distension 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 1 states that the absolute range of movement between deep inspira- 

 tion 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. 



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 

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

 respiratory excursions are smallest. These facts may, perhaps, explain why it is that patients 

 suffering from severe dyspnoea 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 down- 

 ward suction, when the patient sits or stands, and positive, or an upward pressure, when he lies; 

 (3) the intra-abdominal tension due to the abdominal muscles. These are in a state of contrac- 

 tion in the standing position and not in the sitting; hence the diaphragm, when the patient 

 stands, is pushed up higher than when he sits. 



The Intercostales interni and externi have probably no action in moving the ribs. They con- 

 tract simultaneously and form strong elastic supports which prevent the intercostal spaces being 

 pushed out or drawn in during respiration. The anterior portions of the Intercostales interni 

 probably have an additional function in keeping the sternocostal and interchondral joint sur- 

 faces in apposition, the posterior parts of the Intercostales externi performing a similar function 

 for the costovertebral articulations. The Levatores costarum being inserted near the fulcra of 

 the ribs can have little action on the ribs; they act as rotators and lateral flexors of the vertebral 

 column. The Transversus thoracis draws down the costal cartilages, and is therefore a muscle 

 of expiration. 



The Serrati are respiratory muscles. The Serratus posterior superior elevates the ribs and 

 is therefore an inspiratory muscle. The Serratus posterior inferior draws the lower ribs down- 

 ward and backward, and thus elongates the thorax; it also fixes the lower ribs, thus assisting 

 the inspiratory action of the diaphragm and resisting the tendency it has to draw the lower 

 ribs upward and forward. It must therefore be regarded as a muscle of inspiration. 



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

 respiration, and (6) deep respiration. 



Quiet Respiration. The first and second pairs of ribs are fixed by the resistance of the cervical 

 structures; the last pair, and through it the eleventh, by the Quadratus lumborum. The other 

 ribs are elevated, so that the first two intercostal spaces are diminished while the others are 

 increased in width. It has already been shown (p. 304) that elevation of the third, fourth, fifth, 

 and sixth ribs leads to an increase in the antero-posterior and transverse diameters of the thorax; 

 the vertical diameter is increased by the- descent of the diaphragmatic dome so that the lungs 

 are expanded in all directions except backward and upward. Elevation of the eighth, ninth, 

 and tenth ribs is accompanied by a lateral and backward movement, leading to an increase in 

 the transverse diameter of the upper part of the abdomen; the elasticity of the anterior abdominal 



1 Journal of Anatomy and Physiology, 1908, vol. xliii. 



