upon Animal Bodies. 11 



all animal heat is derived from the decomposition of 

 oxygen gas, why is it that irritability is greatest at birth, 

 and gradually lessens as life advances ? The dark co- 

 lour of the placenta will not justify the belief, that much 

 oxygen is received by that organ. Drs. Girtanne. and 

 Fothergill seem both inclined to the opinion, that oxy- 

 gen is the cause of irritability. Great has been the zeal 

 with which physiologists have applied the chemical doc- 

 trines to the solution of a favourite theory. I might re- 

 mark, that the lower tribes of animals (as the polypus, 

 for example), in which respiration is not an immediately 

 vital function, are yet endowed with far superior degrees 

 of irritability than the more perfect animals. Motion, 

 sensation, irritability, heat, and life itself all exist when 

 inspiration begins. It imparts, therefore, no new attri- 

 bute ; it simply commences the motions of an as yet 

 dormant organ, whose action is now essential to the pre- 

 servation of all the above-mentioned properties. It is 

 the pendulum of the machine, whose equal and regular 

 actions harmonize the motions of the whole. Its mo- 

 tion stopped, man becomes a lifeless carcase. Again 

 renewed, again he wakes to intelligence. The power 

 of contraction, by means of which the inspiratory mus- 

 cles are thrown into action, the ribs and sternum elevat- 

 ed, and the cavity of the thorax enlarged in every di- 

 rection, is doubtless superior in the force of its action to 

 the impulse which is the moving agent. In the struc- 

 ture of the thorax, nature has consulted facility of motion. 

 The mere relaxation of the muscles contracted in inspi- 

 ration, aided by the gradual return of the cartilages of 

 the ribs, mediastinum, and lungs from their inherent 

 elasticity, is sufficient ordinarily to expel the air, for, by 



