i6o Mnjcles of Expiration [Book IX, 



the heart. When the diaphragm contrails, .its con- 

 cavity is IdTened, particularly on each fide, over which 

 the lungs are placed, its center being firmly fixed from 

 its connection with the mediaftinum. By the defcent, 

 however, of its fides, it pufhes downwards and for- 

 wards the abdominal vifcera, it lengthens, and of courfe 

 enlarges, the cavity of the thorax, and is the principal 

 mufcleof infpiration. The ribs are at the fame time 

 raifed by the intercoftal mufcles, by which the thorax 

 is made wider. The chief mufcles of expiration, on 

 the other hand, are thofe which furround the abdomen. 

 Thefe counteract the intercoftals, by pulling down the 

 ribs, in which they are afliftcd by the ferrati, and op- 

 pofe the diaphragm by the poftici inferiores prefling 

 backwards and upwards the abdominal vifcera. By 

 thefe mufcles refpiration is in general carried on. - In 

 cafes, however, of laborious refpiration, whether from 

 difeafe or violent exercife, other mufcles ar called into 

 action ; infpiration is then promoted by the pectoral 

 mufcles, the ferrati antici majores, the ferrati poftici 

 pofteriores, and the fcaleni. That thefe mufcles may 

 act with more advantage, perfons labouring under diffi- 

 cult refpiration extend and fix the neck, and raife the 

 fhoulders. In laborious expiration the quadrati lumbo- 

 rum, facro-lumbales, and longifiimi dorfi, concur in 

 pulling down the ribs. The elafticity of the cartilages 

 of the ribs is alfo on all occafions an agent in expi* 

 ration. 



The mufcles fituated on the anterior part of the ab- 

 domen are five pair. On the middle of the anterior part 

 of the abdomen, three of its mufcles, the two oblique 

 and the tranfverfe, terminate in tendinous fubftance, 

 v/hich forms an expanfion the whole way from the car- 

 tilago enfiformis to the ofla pubis. This from its white 

 appearance is calkd linea alba. The external layer is 



formed 



