678 Comparative Morphology of Chordates 



to which the animal is aquatic. All aquatic mammals are wholly air- 

 breathing. The wholly aquatic mammal has little need of smelling 

 air-borne substances and is incapable of smelling anything in the water 

 because, for obvious reasons, it may not inhale water. The olfactory 

 structures are fairly well developed in seals, probably less so in sea 

 cows, and reach the limit of reduction in cetaceans. Mere vestiges of 

 olfactory nerves are found in whalebone whales, and in some toothed 

 cetaceans the olfactory nerves are completely lacking and the olfactory 

 lobes of the brain are reduced. Of the two existing monotremes, the 

 anteater, Echidna, has a well-developed olfactory organ with six or 

 seven turbinals, but in the semiaquatic duckbill there are no ecto- 

 turbinals and the organ is not well developed. 



The vomeronasal (Jacobson's) organ (see p. 475) is especially 

 prominent in monotremes and usually well developed in such "lower" 

 mammals as marsupials, insectivores, rodents, and "edentates." In 

 most other mammals it is usually present as an apparently functional 

 organ, but it is reduced or lacking in some bats and in cetaceans, while 

 in the higher primates, apes and man, it is represented in the adult only 

 by an apparently functionless vestige. The organ consists of a pair of 

 small elongated sacs, more or less encased by cartilage, lying close 

 together at either side of the anterior region of the base of the nasal 

 septum. They are buried in the connective tissue beneath the nasal 

 epithelium. In most cases each sac communicates with the oral cavity 

 by a slender nasopalatine duct which opens on the roof of the mouth 

 through the incisive foramen at the anterior region of the bony 

 palate. Exceptionally, according to some accounts, each sac opens, 

 not into the mouth, but into the adjacent nasal cavity (some rodents — 

 rat, mouse, guinea pig). 



Since the vomeronasal sacs develop as hollow outgrowths from the 

 nasal cavities, they are lined by an epithelium which is of the same 

 general nature as the nasal epithelium. It contains numerous olfactory 

 cells whose nerve-fibers pass into the olfactory nerve. As in reptiles, 

 the wall of the sac is innervated by fibers from the trigeminal nerve (V). 



The nature of the function of the vomeronasal organ is even more 

 problematic in mammals than in reptiles. The nasopalatine duct of 

 the mammals is so narrow as to seem poorly adapted for passage of 

 gases from the mouth to the organ. The probability that the organ 

 serves for smelling substances contained in the mouth is weakened if, 

 in some mammals, the organ has no direct connection with the oral 

 cavity. Further, at least some mammals quite effectively smell food 

 which is in the mouth without the aid of a vomeronasal organ. 



