Chap. 12.] Motkn of the Head. itf 



of tendons as to ferve merely as ligaments. The life of 

 all thefe is to ftrengthen and credit the fpine. 



In the defcription which has been given of the muf- 

 cles which ferve for the motion of the whole head, 

 the reader cannot have failed to obferve, how much 

 more numerous thofe are which are inferted into the 

 back part ef the head, and pull it backwards, than 

 thole which have the oppofite infertion and effect. 

 The reafon of this is, that the center of gravity of the 

 head does not fall on the condyls, on which it is fup- 

 ported, but confiderably farther forward ; from which 

 mechanifm it is evident that the mufcles which pull 

 the head back muft be continually acted againft. 

 Hence, when a perfon. falls afleep, or is affected with 

 the palfy, and the mufcles ceafe to act, the head al- 

 ways falls forwards. By the fpine being thus connected 

 towards the poflerior part of the cranium, more fpace 

 'is allowed for the cavities of the mouth and fauces. 

 Mufcles fituatedou the anterior part of the thorax. 

 After having removed the common integuments of 

 the thorax, we obferve a large mufcle, the pectoralis 

 major, which rifes from the cartilaginous extremities 

 of the fifth and fixth ribs, from almoft the whole 

 length of the fte'rnum, and from near half the anterior 

 part of the clavicle. Its fibres run towards the axilla, 

 and it is inferted into the upper and inner part of the 

 os humeri. Its effect is to move the arm forwards 

 and obliquely upwards towards the fternum. 



Having removed this we come to another layer, 

 which confifts of three mufcles. 



The fubclavius is a fmall mufcle which rifes from 

 the firft rib, and is inferted into the inferior part of the 

 clavicle. Its effect is to pull the clavicle downwards 

 and forwards. 



The pectoralis minor arifes from the upper edge of 



the 



