MUSCLES OF INSPIRATION. 117 



per border of the rib below, are the eleven external intercostal muscles, the 

 fibres of which have an oblique direction from above downward and forward. 

 Attached to the inner borders of the ribs, are the internal intercostals, which 

 have a direction from above downward and backward, nearly at right angles 

 to the fibres of the external intercostals. There are also certain muscles 

 attached to the thorax and spine, thorax and head, upper part of humerus, 

 etc., which are capable of elevating either the entire chest or the ribs. These 

 must act as muscles of inspiration when the attachments to the thorax be- 

 come the movable points. Some of them are called into action during ordi- 

 nary respiration ; others act as auxiliaries when respiration is a little exag- 

 gerated, as after exercise, and are called ordinary auxiliaries ; while others, 

 which ordinarily have different uses, act only when respiration is difficult, 

 and are called extraordinary auxiliaries. 



The following are the principal muscles concerned in inspiration : 



MUSCLES OF INSPIRATION. 

 Ordinary Respiration. 



MUSCLE. ATTACHMENTS. 



Diaphragm Circumference of lower border of thorax. 



Scalenus anticus Transverse processes of third, fourth, fifth and 



sixth cervical vertebrae tubercle of first rib. 



Scalenus medius Transverse processes of lower six cervical vertebras 



upper surface of first rib. 



Scalenus posticus Transverse processes of lower two or three cer- 

 vical vertebrae outer surface of second rib. 



External intercostals Outer borders of the ribs. 



Sternal portion of internal intercostals. .Borders of the costal cartilages. 



Twelve levatores costarum. Transverse processes of dorsal vertebrae ribs, 



between the tubercles and angles. 



Ordinary Auxiliaries. 



Serratus posticus superior Ligamentum nuchae, spinous processes of last cer- 

 vical and upper two or three dorsal vertebrae 



upper borders of second, third, fourth and fifth 

 ribs, just beyond the angles. 



Sterno-mastoideus Upper part of sternum mastoid process of tem- 

 poral bone. 



Extraordinary Auxiliaries. 



Levator anguli scapulae Transverse processes of upper three or four cer- 

 vical vertebrae posterior border of superior 



angle of scapula. 



Trapezius (superior portion) Ligamentum nuchae and seventh cervical verte- 

 bra upper border of spine of scapula. 



Pectoralis minor Coracoid process of scapula anterior surface 



and upper margins of third, fourth and fifth 

 ribs, near the cartilages. 



Pectoralis major (inferior portion) Bicipital groove of humerus costal cartilages 



and lower part of sternum. 



Serratus magnus Inner margin of posterior border of scapula 



external surface and upper border of upper 

 eight ribs. 



