210 MUSCLES OF THE THORAX. 



nsertion, by distinct tendons, into the ribs and spinous processes. Being 

 made up of a number of distinct fasciculi, which alternate in their actions, 

 the spine is kept erect without fatigue, even when they have to counter- 

 balance a corpulent abdominal development. The continuations upwards 

 of these muscles into the neck preserve the steadiness and uprightness of 

 that region. When the muscles of one side act alone, the neck is rotated 

 upon its axis. The complexus, by being attached to the occipital bone, 

 draws the head backwards, and counteracts the muscles on the anterior 

 part of the neck. It assists also in the rotation of the head. 



The semi-spinales and multifidus spince muscles act directly on the ver- 

 tebra, and contribute to the general action of supporting the vertebral 

 column erect. 



The four little muscles situated between the occiput and the two first 

 vertebrae, effect the various movements between these bones ; the recti 

 producing the antero-posterior actions, and the obliqui the rotatory mo- 

 tions of the atlas on the axis. 



The actions of the remaining muscles of the spine, the supra and inter- 

 spinales and inter-transver sales, are expressed in their names. They ap- 

 proximate their attachments and assist the more powerful muscles in pre- 

 serving the erect position of the body. 



The levatores costarum raise the posterior parts of the ribs, and are 

 probably more serviceable in preserving the articulation of the ribs from 

 dislocation, than in raising them in inspiration. 



MUSCLES OF THE THORAX. 



The principal muscles situated upon the thorax belong in their actions 

 to the upper extremity, with which they will be described. They are the 

 pectoralis major and minor, subclavius and serratus magnus. The true 

 thoracic muscles are few in number, and appertain exclusively to the ac- 

 tions of the ribs ; they are, the 



Intercostales externi, 

 Intercostales interni, 

 Triangularis sterni. 



The intercostal muscles are two planes of muscular and tendinous 

 fibres directed obliquely between the adjacent ribs and closing the inter- 

 costal spaces. They are seen partially upon the removal of the pectoral 

 muscles, or upon the inner surface of the chest. The triangularis sterni 

 is within the chest, and requires the removal of the anterior part of the 

 thorax to bring it into view. 



The INTERCOSTALES EXTERNI, eleven on each side, commence poste- 

 rioily at the tubercles of the ribs, and advance forwards to the costal car- 

 tilages, where they terminate in a thin aponeurosis, which is continued 

 onwards to the sternum. Their fibres are directed obliquely downwards 

 and inwards, pursuing the same line with those of the external oblique 

 muscle of the abdomen. They are thicker than the internal iritercostals. 



The INTERCOSTALES INTERNI, also eleven on each side, commence ante- 

 riorly at the sternum, and extend backwards as far as the angles of the 

 nbs, whence they are prolonged to the vertebral column by a thin apo- 

 neurosis. Their fibres are directed obliquely downwards and backwards, 

 and correspond in direction with those of the internal oblique muscle of 



