VESTIGIAL STRUCTURES. 81 



slits of birds, reptiles, and mammals differ from those of 

 fish in that they never at any period support gills. 

 These rudimentary gill-slits further resemble those of 

 the dog-fish, for they present on their anterior aspect a 

 small swelling, or tubercle, representing an operculum 

 In the human embryo four branchial slits present them- 

 selves (fig. 38). 



The first of these represents the spiracle of the shark, 

 and in mammals becomethe tympano-eustachian passage, 

 and is subservient to the sense of hearing ; the small 

 tubercles surmounting it coalesce, and gradually give rise 

 to the pinna, or external ear, so conspicuous in nearly 

 all land mammals. Normally the posterior 

 gill-slits disappear. It is by no means 

 uncommon to find in the sides of the neck 

 of a child, along the anterior border of 

 the sterno-mastoid muscle, small openings 

 in the skin capable of admitting a thin FlG - 38.-An early 



human embryo 



probe. These congenital fistulse, espe- with the bran 

 cially when they exist in the upper part chial slits - 

 of the neck, communicate with the pharynx. This 

 in some cases may be demonstrated by allowing the 

 child to swallow milk ; drops of the milk will find 

 their way through the fistula and appear in the neck. 

 Stress must be placed on this simple experiment, for 

 His, of Leipzic, has urged that branchial fistulae in man 

 never communicate with the pharynx, and that the con- 

 nection, in those which were supposed to open into it, was 

 the result of incautious use of the probe. This view is 

 erroneous ; I have seen milk issue from such fistulae in 

 individuals who have never been submitted to sounding 



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