292 LAY SERMONS, ESSAYS, AND REVIEWS. [xiv. 



their appropriate nervous centres. The even rhythm of the 

 breathing of every one of us depends upon the structural 

 integrity of a particular region of the medulla oblongata, as 

 much as the ticking of a clock depends upon the integrity 

 of the escapement. You may take away the hands of a clock 

 and break up its striking machinery, but it will still tick ; and 

 a man may be unable to feel, speak, or move, and yet he will 

 breathe. 



Again, in entire accordance with Descartes affirmation, it is 

 certain that the modes of motion which constitute the physical 

 basis of light, sound, and heat, are transmuted into affections of 

 nervous matter by the sensory organs. These affections are, so 

 to speak, a kind of physical ideas, which are retained in the 

 central organs, constituting what might be called physical 

 memory, and may be combined in a manner which answers to 

 association and imagination, or may give rise to muscular con 

 tractions, in those &quot; reflex actions &quot; which are the mechanical 

 representatives of volitions. 



Consider what happens when a blow is aimed at the eye. 1 

 Instantly, and without our knowledge or will, and even against 

 the will, the eyelids close. What is it that happens ? A picture 

 of the rapidly advancing fist is made upon the retina at the back 

 of the eye. The retina changes this picture into an affection of 

 a number of the fibres of the optic nerve ; the fibres of the optic 

 nerve affect certain parts of the brain ; the brain, in consequence, 

 affects those particular fibres of the seventh nerve which go to 

 the orbicular muscle of the eyelids ; the change in these nerve- 

 fibres causes the muscular fibres to change their dimensions, so 

 as to become shorter and broader ; and the result is the closing 

 of the slit between the two lids, round which these fibres are 

 disposed. Here is a pure mechanism, giving rise to a purposive 

 action, and strictly comparable to that by which Descartes sup 

 poses his waterwork Diana to be moved. But we may go further, 

 and inquire whether our volition, in what we term voluntary 



i Compare &quot; Traite des Passions,&quot; Art. XIII. and XVI. 



