268 



COMPARATIVE ANATOMY 



which communicates with the pharynx by the posterior nostrils, 

 the turbinals arise from the lateral Avails of the chamber and are 

 developed later than the ethmoid turbinals : their skeletal frame- 

 work unites with the maxillary bone, while a less complicated 

 ridge, which may unite with the nasal bone, can usually be 

 recognised. The ethmoturbinals project forwards between the 

 nasotnrlinals and maxillpturbina,ls\ the two last-mentioned no longer 

 possess an olfactory epithelium, and have plainly undergone a 

 change of function in connection with the perception of the 

 warmth and moisture of the inspired air. When well-developed, 

 the maxilloturbinal forms a single or double coil, and may even 

 be more or less branched (Fig. 197) ; fibres of the maxillary division 

 of the trigeminal supply its mucous membrane. 



The ethmoturbinals referred to above (endoturbinals) are peculiar 

 to Mammals, as are also certain accessory folds situated laterally 

 to them and also belonging to the ethmoid (Fig. 198) : these may 



c 



D 



E 



J 



Fi<;. 197. -VARIOUS FORMS OF THE MAXILLOTURBINAL BONE IN MAMMALS. 



A, double coil ; _/>, transition from latter to single coil, E, F ; C, transition from 

 double coil to the dendritic form D. (After Zuckerkandl.) 



be described as the postero-lateral or ectoturbinals to distinguish 

 them from the endoturbinals and from the antero-lateral maxillo- 

 turbinals and nasoturbinals, which correspond to those of the 

 Sauropsida. 



The ethmoid turbinals are arranged in a row more or less 

 parallel or obliquely to the palate : their number and relative 

 development varies considerably amongst Mammals and is propor- 

 tionate to the development of the olfactory lobes and sense of 

 smell. In Monotremes two extreme types are seen : Echidna 

 possesses a highly developed and complicated labyrinth of six or 

 more scrolls, while in Ornithorynchus the labyrinth is greatly 

 reduced in adaptation to an aquatic mode of life. A definite type 

 with five endoturbinals occurs in Marsupials, and this may be 

 taken as primarily typical for the Eutheria : it is approached most 

 nearly in Insectivores, in which there are from four to six, and a 

 very similar condition is seen in Hyrax, Bats, Carnivores, Rodents, 



