422 THE MUSCULATURE 



Nerve-supply. — These muscles are all supplied by the posterior branch of the suboccipital 

 (first cervical) nerve. The branch to the two rectus muscles passes across the dorsal surface 

 of the major rectus and supphes branches to the middle of the dorsal surface of each muscle. 

 The branch to the superior oblique muscle enters the middle of the medial margin, that to 

 the inferior obhque about the middle of its superior margin. The inferior oblique and major 

 rectus muscles usually, the other muscles occasionally, receive branches from the second cervical 

 nerve. 



Relations. — The two oblique muscles with the rectus major serve to bound a small tri- 

 angular space, the suboccipital triangle, through which pass the dorsal division of the sub- 

 occipital nerve and the vertebral artery. The two minor recti lie on the atlanto-occipital 

 membrane in the upper part of the space bounded by the major recti. The muscles are covered 

 medially by the semispinalis capitis (complexus), laterally by the longissimus and splenius 

 capitis. In front of the two oblique muscles and the major rectus runs the vertebral artery. 

 The great occipital nerve runs between the semispinalis capitis (complexus) and the inferior 

 obhque and the two recti in a dense fatty connective tissue containing the extensive sub- 

 occipital venous plexus. 



Action. — The rectus muscles and the superior oblique draw the head backward. The 

 rectus major and the inferior oblique, when acting on one side, rotate the face toward that side. 



Variations. — Each of these muscles may be doubled by longitudinal division. Accessory 

 slips may connect the two recti with the semispinalis capitis. The atlanto -mastoid is a small 

 muscle frequently found. It passes from the transverse process of the atlas to the mastoid 

 process. 



IV. THE THORACIC-ABDOMINAL MUSCULATURE 



The thoracic and abdominal viscera are contained within cavities, the ventro- 

 lateral walls of which may be contracted and expanded by muscular action. The 

 skeletal support for the intrinsic musculature of these walls consists of the ribs, 

 the sternum and the vertebral column and the pelvis. The intrinsic musculature 

 in the thoracic walls is situated chiefly between the ribs (intercostal muscles, figs. 

 385, 386) while in the region of the abdomen it extends in broad sheets from the 

 lower part of the thorax to the pelvis (the quadratus lumborum and the external 

 and internal oblique, transverse, and redws muscles, figs. 387, 388, 390, 406). Be- 

 tween the two cavities, attached to the lower part of the thorax and to the lumbar 

 vertebrae lies the dome-shaped diaphragm (fig. 391). The thoracic cavity ex- 

 tends on each side slightly above the first rib. The abdominal cavity extends 

 downward and backward into the pelvis, as the pelvic cavity. 



The function of the intercostal muscles is to expand and contract the thoracic cavity for 

 the sake of respiration. The shape of the ribs and their articulations with the vertebrae are 

 such that a slight rotation of the neck of each rib will cause the shaft to swing outward and up- 

 ward or in the reverse direction. The costal cartilages are elastic enough to permit this move- 

 ment, and at the same time are strong enough to make the thorax an effective skeletal apparatus. 

 Ninety joints are called into play in the movements of the thorax (24 between the heads of 

 the ribs and the vertebrae, 20 between the tuberosities and the transverse processes of the ver- 

 tebrae, 24 between the ribs and costal cartilages, 14 between the costal cartilages and the sternum, 

 6 between the costal cartilages and 2 intrasternal). When the shafts of ribs are swung outward 

 and upward the thorax is enlarged in the antero-posterior and transverse axes. In the adult 

 when standing the sternum may be raised nearly 3 cm., and protruded 1 cm. The cartilages 

 of the lower ribs may be raised 4 to 5 cm. The side of the thorax at the level of the second 

 rib may be protruded 3 cm., and at the level of the eighth rib nearly as far. This extent of 

 movement, however, is found only in forced respiration. In ordinary quiet respiration it is 

 far less, the sternum being raised merely 3 or 4 mm. and protruded 2 mm., and the thorax is 

 enlarged at the side merely 5 mm. (R. Fick). The chief muscles used in quiet inspiration are 

 the external intercostals and the intercartilaginous parts of the internal intercostals. 



During inspiration the diaphragm contracts so that the thoracic cavity is further enlarged 

 perpendicularly. The extent of movement of the upper part of the diaphragm is estimated by 

 K. Fick at from l|-3 cm. 



The ventro-lateral abdominal muscles contract the thoracic cavity by depressing the 

 thorax and by pushing the diaphragm upward. They directly contract the abdominal cavity. 

 Contraction of the abdominal cavity is of aid in defecation and parturition. The abdominal 

 muscles are also of value in flexion, abduction, and rotation of the vertebral column and pelvis. 



The thorax, with its intrinsic musculature, is in large part covered by the musculature 

 which extends from the trunk to the shoulder girdle and arm; dorsally by the trapezius and 

 rhomboids, vcntraljy by the pectoral muscles, and laterally by the serratus anterior and the 

 latissimus dorsi, as well as by the scapula and the muscles which pass from it to the humerus. 

 The upper extremity on each side is largely supported from the spine by the trapezius, rhomboid 

 and levator scapula; muscles but it none the less exerts some pressure on the thorax and inter- 

 feres to some extent with respiration. If the girdle and arm are fixed or raised the muscles 

 which pass from them to the thorax are an aid in forced inspiration. Advantage of this is 

 taken when in artificial respiration the arms arc raised so as to Uft the ribs through traction 

 by the latissimus dorsi, the pectoralis muscles and the subclavius. Some of the muscles of the 

 neck, especially the scalene mu.sclcs and the sterno-cleido-mastoid, are likewise of value in forced 

 inspiration. 



