Mechanism Of Respiration
The respiratory movements must be examined during (a) quiet respiration; and (b) 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 them 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 that elevation of the third, fourth, fifth and sixth ribs leads to an increase in the anteroposterior 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 backwards and upwards. 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 wall allows a slight increase in the anteroposterior diameter of this part., and in this way the decrease in the vertical diameter of the abdomen is compensated and space provided for its displaced viscera. Expiration is effected by the elastic recoil of the thoracic walls, and by the action of the abdominal muscles which push back the displaced abdominal viscera.
Deep respiration.-All the movements of quiet respiration are here carried out, but to a greater extent. In deep respiration the shoulders and the vertebral borders of the scapulae are fixed and the Trapezius, Serratus anterior, Pectoral, and Latissimus dorsi muscles are called into play. The Scaleni are in strong action; and the Sternocleidomastoids also assist when the head is fixed, by drawing tip the sternum, and by fixing the clavicles. The first ribs are therefore no longer stationary, but are raised with the sternum; with them all the other ribs except the last are raised to a higher level. In conjunction with the increased descent of the Diaphragm this provides for a considerable augmentation of all the thoracic diameters. The anterior abdominal muscles come into action so that the umbilicus is drawn upwards and backwards, but this allows the Diaphragm to exert a more powerful influence on the lower ribs; the transverse diameter of the upper part of the abdomen is greatly increased and the infrasternal angle opened out. The deeper muscles of the back, e.g. the Serrati posteriores superiores and the Sacrospinales and their continuations, are also brought into action; the thoracic curve of the vertebral column is partially straightened, and the whole column, above the lower lumbar vertebrae, drawn backwards. This increases the anteroposterior diameters of the thorax and upper part of the abdomen and widens the intercostal spaces. Deep expiration is effected by the recoil of the thoracic walls and by the contraction of the anterolateral muscles of the abdominal wall.
Figure 523
Halls Dally gives the following figures as representing the average changes which occur during deepest possible respiration. The manubrium sterni moves 30 mm in an upward and 14 mm, in a forward direction; the width of the infrasternal angle, at a level of 30 mm. below the articulation between the body of the sternum and the xiphoid process, is increased by 26 mm; the umbilicus is retracted and drawn upwards for a distance of 18 mm.
Applied Anatomy-The changes in the height of the Diaphragm during alterations in posture explain why patients suffering from severe dyspnia are most comfortable and least short of breath when they sit up. In unilateral disease of the pleura or lungs interference with the position or movement of the Diaphragm can generally be observed skiagraphically. Middleton, by estimating the vital capacity in cases where the action of the Diaphragm was unpaired by thoracic wounds or empyema, concludes that the normal diaphragmatic contraction is responsible for 60 per cent, of the respiratory exchange in deep breathing.
Figure 524-525
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Vertebral Column