Mr. Mayo on Human Phydology. 97 



can, by an effort, preserve a perfect serenity of features. 

 In certain states of mind, we are instinctively led to throw- 

 certain muscles into action ; but suppose that we feel the 

 expediency of concealing our emotions, upon this stronger 

 motive we give the features a different character. 



Habitually we breathe with a certain degree of frequency 

 upon instinct ; in other words, we draw our breath not 

 upon a specific calculation of what we shall gain by it, but 

 from an innate tendency to do so ; but if we have an ade- 

 quate object proposed, we breathe at a different rate : and 

 if we have breathed for a short time by a conscious effort of 

 the will with measured inspirations, we allow the function 

 to return to its usual rate without any abrupt transition, as 

 from one mode of action to a different mode. 



It further deserves to be considered, that although some 

 parts may more commonly act upon instinct than others, yet 

 no distinction exists in the muscular frame between parts 

 that act on instinctive impulses, and parts which act only 

 on measured and conscious instigations of the will. The 

 muscles of the face commonly play instinctively, but during 

 violent emotion the whole frame partakes of the same un- 

 conscious expression. 



And what is perhaps still more conclusive, — in those parts, 

 the muscles of which act alternately upon instinct or other- 

 wise, one nerve alone regulates equally each mode of action. 

 Mr. Mayo mentions having ascertained, upon dividing the 

 portio dura of the seventh nerve on each side of the head of 

 an ass, that the muscles of its face were absolutely and 

 entirely paralyzed for every kind of movement. 



In the account of the special senses, the phenomena of 

 vision are elaborately explained. As extracts from this 

 chapter, we shall give Mr. Mayo's account of the rationale 

 of vision by inverted images, and of the nature of the action 

 of the iris. 



Mr. Mayo quotes the following experiments made by 

 Scheiner from Dr. Reid. 



1. If the head of a pin strongly illuminated be held at 

 the distance of four inches from the eye, the object appears 

 large and imperfectly defined. If a card be then interposed, 

 pierced with a pinhole, the object when seen through the 

 pinhole appears distinct at the same distance. If the inter- 

 posed card be pierced with two pinholes a line asunder, two 

 objects are seen instead of one ; and on closing one pinhole, 

 the object referred to the opposite side vanishes. In other 

 words, when an object is painted upon two distinct points 



JAN.— MARCH, 1827. H 



