172 THE STRUCTURE OF MAN 
which are destitute of branchial organs, and of these (eg. in the 
Lizard) only the three anterior, as a rule, break through the outer — 
integument. The fourth, in exceptional cases, may also break 
through, but this never occurs with the fifth. The same is the 
case in Birds, except that in them the third pair of sacs open 
externally only in exceptional cases, and that the fourth and 
fifth pairs, which are inconstant in their appearance, never break 
through. In Mammals and Man only four pairs of branchial 
sacs arise, and here also those which lie most posteriorly are 
decidedly vestigial in character. For this reduction a parallel 
is forthcoming in the branchial apparatus of the Anamnia ; 
and there is thus evidence both in Phylogeny and in Ontogeny 
of a progressive suppression of the branchial pouches and arches 
in postero-anterior succession. 
The branchial pouches and the skeletal arches which support 
them thus belong, in the higher Vertebrata and Man,’ in which 
they never bear functional respiratory organs, to the category of 
typical vestigial structures [inherited and for the most part lost 
—unintelligible, as Gegenbaur long ago insisted, except in the 
knowledge, furnished by comparative morphology, that in certain 
lower animals their full development is indispensable to exist- 
ence}. 
There occasionally occur in the anterior cervical region in 
Man “ fistulee,” which may penetrate a greater or lesser distance 
in from the integument, or may bound canals which even open 
into the pharynx. These are abnormal structures, due to arrested 
development, under which branchial clefts have not become com- 
pletely obliterated. In dealing with the auditory organ details 
have already been given (ante, p. 150) of the relationship of 
the cavity of the middle ear (Eustachian tube) to the modified 
remnant of the first visceral cleft, which in the higher Vertebrata 
has undergone a new development, in adaptation to a change of 
function. | 
THE LARYNX 
The study both of the innervation of the musculature of the 
larynx, and of the genesis and Comparative Anatomy of its 
cartilaginous framework, strongly suggest its origin, for the 
1 The branchial sacs, and the external branchial furrows in the outer integument 
which correspond with them, are most distinctly visible in human embryos of 3-4 
mm. in length. 
