Chap. VII] MUSCULAR TISSUE 105 



Pectoralis minor. — The pectoralis minor is underneath and 

 entirely covered by the major. It arises from the surface of the 

 third, fourth, and fifth ribs, near the cartilages, and is inserted in 

 the coracoid^ process of the scapula. 



Action. — It pulls the shoulder downward and assists in the 

 elevation of the ribs during inspiration. 



Muscles of the thorax. — The muscles of the thorax are chiefly 

 concerned with the movements of the ribs during respiration. 

 They are the: (1) intercostals, and (2) levatores costarum. 



Intercostals. — The intercostals are found filling the spaces be- 

 tween the ribs. Each muscle consists of two layers, one ex- 

 ternal and one internal, and as there are eleven intercostal spaces 

 on each side, and two muscles in each space, it follows there 

 are forty-four intercostal muscles. The fibres of these muscles 

 run in opposite directions. 



External intercostals. — The external fibres arise from the lower 

 border of a rib, run downward, forward, and inward, and are in- 

 serted into the upper border of the next lower rib. 



Action. — They pull the ribs upward and outward, thereby in- 

 creasing the chest cavity. 



Internal intercostals. — The internal fibres arise from the lower 

 border of a rib, run downward, outward, and backward, and are 

 inserted into the upper border of the next lower rib. 



Action. — They depress the ribs. 



Levatores costarum (lifters of the ribs). — They arise from the 

 transverse processes of the vertebrae from the seventh cervical to 

 the eleventh thoracic, and are inserted into the outer surface of 

 the ribs between the tubercles and angles. 



Action. — They assist in elevation of the first ten ribs, and with 

 other muscles draw the lower ribs backward. 



Diaphragm. — The diaphragm is a thin, musculo-fibrous par- 

 tition placed obliquely between the abdominal and thoracic 

 cavities. It is dome-shaped, and consists of muscle fibres arising 

 from the whole of the internal circumference of the thorax, and 

 of a central aponeurotic tendon, shaped somewhat like a trefoil 

 leaf, into which the muscle fibres are inserted. It has three large 

 openings for the passage of the aorta, the large artery of the body, 

 the inferior vena cava, one of the largest veins of the body, and the 



1 See Fig. 49 for eoracoid process of scapula. 



