PHYSIOLOGY. 



the nervous fluid, or of the blood into the muscles concerned in 

 these functions. 



Inspiration, or the action of the muscles producing it, is ex- 

 cited, in all cases of general effort, to remove pain and uneasi- 

 ness ; and it is perhaps a propensity of this kind that gives the 

 first beginning to respiration in the new-born infant exposed to 

 several new and uneasy impressions. 



For the continuance of respiration, inspiration is especially 

 excited by the sense of uneasiness that attends any difficulty in 

 the passage of the blood through the vessels of the lungs ; but 

 this uneasiness arises, in some measure, at the end of every ex- 

 piration, and is much increased by any continuance of this state. 



CXCIV. Expiration, in some measure, necessarily succeeds 

 inspiration, by the spontaneous relaxation of the inspiratory mus- 

 cles (CVIIL), while the elasticity of the membranes, ligaments, 

 and cartilages, stretched in inspiration, brings back the ribs and 

 diaphragm into their former situations ; and the same effects are 

 also produced by the action of the abdominal muscles, and of 

 the muscular fibres of the bronchiae; both of which are stretched, 

 and thereby excited in inspiration. 



In the case of ordinary inspiration these causes are sufficient 

 to produce a spontaneous expiration. But as it appears that 

 a violent and long continued inspiration interrupts the passage 

 of the blood through the lungs, this creates an uneasiness, and 

 a propensity, which must produce a relaxation of the inspira- 

 tory, and excite a contraction of the expiratory muscles. 



It is further to be supposed, that, in animals which have 

 breathed for some time, custom has associated the several mo- 

 tions concerned both in inspiration and expiration ; so that an 

 irritation applied to any part of them, necessarily excites the 

 whole ; and it may also be supposed, that habit determines these 

 motions regularly to succeed one another. 



CXCV. In this manner (CXCIII. CXCIV.) respiration is 

 continued for the general purposes of the animal economy ; but 

 the several motions of which it consists are also occasionally ex- 

 cited, and variously modified by the will, intending particular 

 effects to be produced by these motions. They are also excited, 

 and variously modified by certain emotions and passions, and 

 give particular expressions of these. They are often excited 



