THE DIGESTIVE AND RESPIRATORY SYSTEM 269 



then for a very short period. The most anterior of these, 

 however, which, in selachians, appears as the spiraculum, or 

 blow-hole, persists in all higher vertebrates, as the Eustachian 

 tube [tuba auditiva BNA~\ and the middle ear. The remain- 

 ing pockets disappear as such, but various accessory structures, 

 such as cartilages, muscles, arteries, and glands, arise in the 

 embryo in association with them and afterwards become modi- 

 fied to subserve numerous important purposes.* A not uncom- 

 mon malformation in man, a few cases of which have been 

 reported in other mammals, is that of a cervical fistula, which 

 forms an open communication between pharynx and exterior, 

 usually upon one side alone. This is nothing more or less 

 than a permanent gill-slit and may be considered as a case of 

 arrested development, or the retention of what is designed to 

 be a transitory stage. 



The nasal cavities, which lie above the anterior part of 

 the stomato-pharyngeal cavity, are in fishes quite independent 

 of the latter, but come into direct communication with it in 

 Amphibia by the formation of a pair of openings, the posterior 

 nares or choance, which appear in the roof of the mouth. This 

 communication was apparently one of the changes inaugurated 

 during the transition from water to land, and allows the in- 

 gress and egress of air to the pharynx and thence to the lungs 

 without opening the mouth, since this action, although harm- 

 less for an animal immersed in water, would soon cause the 

 drying up of the mucous membrane lining the mouth cavity 

 if resorted to in air with anywhere near the same frequency. 

 In the case of the nasal cavities this is prevented in part by the 

 small size of the external openings, but still more by the for- 

 mation of slime glands capable of producing an abundant se- 

 cretion. The waste lacrimal fluid conveyed from the eyes 

 to the nose is undoubtedly also of assistance in this respect. 



The posterior part of the pharynx shows a strong tendency 

 to form median diverticula, either dorsal or ventral, which ex- 



* Cf. Chap. V under Visceral skeleton ; Chap. VI under Visceral mus- 

 cles, Chap. VIII under Arterial arches; and the present chapter farther 

 on under Thymus and Thyreoid. 



