470 THE MUSCULAR SYSTEM. 



The superior oblique portion arises from the anterior tubercles of the transverse 

 processes of the third, fourth, and fifth cervical vertebrae. 



It is directed upwards, to be inserted into the anterior tubercle of the atlas. 



Nerve-Supply. It is supplied by nerves from the anterior rami of the second, third, and 

 fourth cervical nerves. 



Action. A flexor of the vertebral column. 



M. Rectus Capitis Lateralis. The rectus capitis lateralis, in series with 

 the posterior inter-transverse muscles in the neck, arises from the transverse 

 process of the atlas. 



It is inserted into the inferior surface of the jugular process of the occipital 

 bone. It is placed alongside the rectus capitis anterior, separated from it by the 

 anterior ramus of the first cervical nerve. 



Nerve-Supply. The loop between the anterior rami of the first two cervical nerves. 



Actions. A lateral flexor of the head and vertebral column. The movements produced by 

 these muscles are considered along with those of other muscles acting on the head, vertebral 

 column, and thorax (pp. 445, 446). 



THE MUSCLES OF THE THORAX. 

 Muscles of Respiration. 



The muscles which complete the boundaries of the thorax are the diaphragm 

 and intercostal muscles (external and internal), along with three series of smaller 

 muscles the transversus thoracis, the levatores costarum, and the subcostal 

 muscles. 



Mm. Intercostales. The intercostal muscles are arranged in eleven pairs, 

 which occupy the intercostal spaces. 



Each external muscle arises from the sharp lower border of a rib, and 

 is directed inferiorly and anteriorly, to be inserted into the external edge of the 

 superior border of the rib below. It extends from the tubercle of the rib posteriorly 

 nearly to the costal cartilage anteriorly. The anterior intercostal aponeurosis is 

 continuous with it anteriorly, and extends forwards to the side of the sternum. 



Each internal muscle arises from the costal cartilage and the internal or 

 superior edge of the costal groove, and is directed inferiorly and posteriorly, to be 

 inserted into the internal edge of the superior border of the rib and costal cartilage 

 below. It extends from the side of the sternum anteriorly to the angle of the rib 

 posteriorly, where it is replaced by the posterior intercostal aponeurosis extending 

 to the tubercle of the rib. 



The superficial surface of the external muscle is covered by the muscles of 

 the chest, axilla, abdomen, and back. The deep surface of the internal muscle is in 

 contact with the pleura. 



Mm. Levatores Costarum. The levatores costarum are in series with the 

 external intercostal muscles. They are twelve small slips arising from the trans- 

 verse processes of the seventh cervical and upper eleven thoracic vertebrae. Each 

 spreads out in a fan-like manner as it descends to the lateral surface of the rib 

 immediately below where it is inserted posterior to the angle. 



Mm. Subcostales. The subcostal muscles are slips of muscles found on the 

 internal surface of the lower ribs near their angles. They are in series with the 

 internal intercostal muscles, but pass over the deep surface of several ribs. 



M. Transversus Thoracis. The transversus thoracis (O.T. triangularis 

 sterni) occupies the posterior aspect of the anterior thoracic wall, and is separated 

 from the costal cartilages by the internal mammary vessels. It arises from the 

 posterior surface of the xiphoid process and body of the sternum as high as the 

 level of the third costal cartilage. 



From that origin its fibres radiate laterally, the lower horizontally, the upper 

 fibres obliquely upwards, to be inserted into the second, third, fourth, fifth, and 

 sixth costal cartilages. The muscle is continuous below with the transversus 

 abdominis. 



