172 THE STRUCTURE OF MAN 



which are destitute of branchial organs, and of these (e.g. 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, 1 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 " fistulae," 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 thejirst 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. 



