33^ THE EVOLUTION OF MAM. 



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Comparative Anatomywe can trace the progressive develop 

 ment of this important organ from its very simple rudiment 

 in the lower Amphibia up to the complex and vocal appara- 

 tus represented by the larynx of Birds and Mammals. 



Though these organs of voice, speech, and air-respiration 

 develop so differently in the various higher Mammals, they 

 yet all arise from the same simple original rudiment — 

 from a vesicle which grows out of the wall of the anterior 

 intestine. We have thus satisfied ourselves of the interest- 

 ing fact that both the respiratory apparatus of Vertebrates 

 develop from the fore part of the intestinal canal ; first, the 

 primary and more primitive water- respiring apparatus, the 

 gill-body, which is altogether lost in the three higher 

 vertebrate classes ; and, afterwards, the secondary and more 

 recent air-breathing apparatus, which acts in Fishes only 

 as a swimming-bladder, but as a lung from the Dipneusta 

 upwards. 



We must say a few words about an interesting rudi- 

 mentary organ of the respiratory intestine, the thyroid 

 gland (thyreoidea), the large gland situated in front of the 

 larynx, and below the so-called " Adam's apple," and which, 

 especially in the male sex, is often very prominent ; it is 

 produced in the embryo by the separation of the lower wall 

 of the throat (pharynx). This thyroid gland is of no 

 use whatever to man; it is only aesthetically interesting, | 

 because in certain mountainous dis^xicts it has a tendency 

 to enlarge, and in that case it forms the " goitre " which 

 hangs from the neck in front. Its dysteleological interest 

 is, however, far higher ; for as Wilhelm Miiller of Jena 

 has shown, this useless and unsightly organ is the last 

 remnant of the " hypobranchlal groove," which we hiiv© 



