35 



No study carries along v?ith it a more lively interest, than that of the 

 admirable relations existing between the conformation of our parts and 

 the external objects, to which they are applied. These relations are cal- 

 culated with such precision, and laid down with such accuracy, that the 

 organs of sense and of motion, considered in this point of view, may be 

 regarded as the model of the most ingenious productions of art. So true 

 it is in the words of the great physician of Pergamus, that nature did 

 every thing before art, and better*. 



At the beginning of the last century, geometrical physicians, deceived 

 by an appearance of rigid precision, attempted to explain every thing by 

 the calibre of vessels, their length, their curvatures, the compound ratio 

 of the action of solids and the impulse of fluids. Hence were formed theo- 

 ries so very imperfect, that, as we shall see, in treating of several points 

 of physiology, and especially of the force with which the heart acts, not 

 one of those who proposed them, coincides with those who have since 

 followed their track. However, it does not admit of a doubt, that there 

 occur in the animal machine, effects which are referable to the laws of 

 hydraulics. The brain, for example, required a large and constant sup- 

 ply of arterial blood, vivified by recent circulation through the lungs; but 

 the too rapid and abrupt access of that fluid in the brain, might have dis- 

 ordered its structure. Nature, therefore, has, as we shall mention in the 

 article of the cerebral circulation, employed all the hydraulic resources 

 in her power, to break the force Avith which the blood enters the brain, 

 and to slacken its course. 



Has man ever applied the laws of hydraulics in a more felicitous man- 

 ner than nature, in the rete mirable formed at the base of the brain by the 

 carotids of quadrupeds? An arrangement truly remarkable, without which, 

 the blood conveyed to the brain by those arteries, impelled by a force su- 

 perior to that of th*^ human heart, and not having to overome the resist- 

 ance of its own gravity, would infallibly have occasioned a disorganiza- 

 tion of that organ, whose consistence is so soft. 



As to the application which is allowable of mathematical sciences, it 

 maybe said, that, as in physiology, but little is absolutely certainf, and 

 much merely probable, we can reckon only on probabilities, and seek our 

 elements in facts deduced from observation or experience ; facts which 

 when collected and multiplied to a certain degree, lead to results of equal 

 value with truths absolutely demonstrated. 



The phenomena presented by living bodies vary incessantly, in their 

 activity, their intensity, and their velocity. How can mathematical for- 

 mulae apply to such variable elements? As well might you inclose in a 

 frail vessel, hermetically sealed, a fluid subject to expansion, and of vari- 

 able bulk. The motions of progression in man and in the animals, afford, 



them. Pathology and the treatment of diseases, also, reflect a light upon physiology 

 uhich they first derived from this productive source. 



* Quandoquidem natura, nt arbitror, et prior tempore sit, et in operibus magis sapiens 

 qumnars. GALENUS, de usu partiwn, lib. VII. cap. 13. 



It was from observing the manner in which nature prevents the diffusion of light in 

 the globe of the eye, that Euler was led to the improvement of his astronomical teles- 

 cope luthor's Note. 



j This is to be understood, as applying only to the causes of the phenomena, and not 

 to the phenomena themselves; for physiology is perhaps richer than any other science, 

 in facts unquestionable, and easily ascertained by observation. Auih'ir\ -Vote. 



