424 THE MUSCLES AND FASCIAE 



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

 resistance of the cervical structures; the last pair, and through it the eleventh pair, are fixed 

 by the Quadratus lurnborum muscles. The other ribs are elevated, so that the first two inter- 

 costal spaces are diminished while the other spaces are increased in width. It has already 

 been shown (p. 285) 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 

 an outward 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 antero-posterior 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 abdominal walls and by the action of the abdominal muscles, 

 which push back the viscera displaced downward by the Diaphragm. 



Forced Respiration. All the movements of quiet respiration are here carried out, but to 

 a greater extent. In inspiration the shoulders and the vertebral borders of the scapulse are 

 fixed and the limb muscles, Trapezius, Serratus magnus, Pectorals, and Latissimus dorsi, are 

 called into play. The Scaleni are in stronger action, and the Sternomastoids also assist, when 

 the head is fixed, by drawing up the sternum and by fixing the clavicles. The first rib is there- 

 fore no longer stationary, but, with the sternum, is raised; with it 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 abdom- 

 inal muscles come into action so that the umbilicus is drawn upward and backward, and 

 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 subcostal angle is opened 

 out. The deeper muscles of the back, e. g., the Serrati postici superiores and the Erectores 

 spinae, are also brought into action; the thoracic curve of the vertebral column is partially straight- 

 ened, and the whole column, above the lower lumbar vertebrae, is drawn backward. This 

 increases the antero-posterior diameters of the thorax and of the upper part of the abdomen and 

 widens the intercostal spaces. Forced expiration is effected by the recoil of the abdominal walls, 

 by the contraction of the antero-lateral muscles of the abdominal wall, and of the Serrati postici 

 inferiores and Triangularis sterni. 



Halls Dally (op. dt.) gives the following figures as representing the average changes which 

 occur during deepest possible respiration. The manubrium moves 30 mm. in an upward, and 

 14 mm. in a forward direction; the width of the subcostal angle, at a level of 30 mm. below 

 the articulation of the manubrium with the gladiolus, is increased by 26 mm.; the umbilicus 

 is retracted and drawn upward for a distance of 13 mm. 



III. MUSCLES OF THE ABDOMEN. 



The muscles of the abdomen may be divided into two groups: (1) The 

 antero-lateral muscles of the abdomen. (2) The posterior muscles of the abdomen. 



1. The Antero-lateral Muscles of the Abdomen. 



The muscles of this group are the 



External oblique. Transversalis. 



Internal oblique. Rectus. 



Pyramidalis. 



Dissection (Fig. 318). To dissect the abdominal muscles, make a vertical incision from the 

 ensiform cartilage to the symphysis pubis; a second incision from the umbilicus obliquely upward 

 and outward to the outer surface of the thorax, as high as the lower border of the fifth or sixth 

 rib; and a third, commencing midway between the umbilicus and pubes, transversely outward 

 to the anterior superior iliac spine, and along the crest of the ilium as far as its posterior third. 

 Then reflect the three flaps included between these incisions from within outward, in the lines 

 of direction of the muscle fibres. If necessary, the abdominal muscles may be made tense 

 Joy inflating the peritoneal cavity through the umbilicus. 



