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 
allland 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 F!6. 38.—An early 
™ human embryo 
probe. These congenital fistula, espe- with the bran 
cially when they exist in the upper part "a! 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 fistula 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 fistule in 
individuals who have never been submitted to sounding 
7 
