MAMMALS. 363 



The olfactory organ is noticeable for the great increase of 

 the olfactory epithelium and the corresponding complexity of 

 the bony labyrinth which supports it. In the formation of this 

 labyrinth small bony processes play the greater part, the upper 

 (superior turbinal) being an outgrowth from the ethmoid, the 

 lower (inferior turbinal) usually uniting with the maxillary. 

 The two cavities are separated behind by the ethmoid and 

 vomer, the partition being continued to the tip of the nose by 

 cartilage. Connected with the nasal cavities are numerous 

 sinuses in the frontal, maxillary, and sphenoid bones. The ol- 

 factory nerve breaks up into numerous fibres before leaving the 

 cranial cavity, and these pass through the perforations in the 

 cribiform plate of the ethmoid. 



The eyes vary according to the habits, being small in bur- 

 rowing forms, or even occasionally without muscles and beneath 

 the skin {Spalax, Chrysochloris). As a rule the eyeball is ap- 

 proximately spherical, except in the whales, where it is flattened. 

 It is placed in orbits, usually incomplete, and these are more or 

 less lateral in position. Sclerotic bones are never developed. 

 Besides the upper and lower lids, a nictitating membrane is 

 usually well developed, but sometimes, as in man, this is reduced 

 to a small muscleless fold, the plica semilunaris, at the inner 

 angle of the eye. In the sirenia the eyelids act as an iris-like 

 diaphragm. Frequently a seventh muscle of the eye, a re- 

 tractor bulbi, is present, and in the carnivores this is four-divided. 

 In carnivores, dolphins, ungulates, and some marsupials, a metal- 

 lic lustre is developed in a part of the choroid (tapetum). 



The ears are marked externally, except in monotremes, ce- 

 tacea, and some seals, by the development of a conch, supported 

 by cartilage, and moved by appropriate muscles. From this the 

 external meat us leads inward to the tympanum, which is crossed 

 by the three ossicula auditus malleus, incus, and stapes al- 

 ready mentioned (p. 358). From the tympanum the Eustachian 

 tube (p. 73) leads to the pharynx, except in the whales, where 

 it enters the nasal passages. The inner ear is characterized 

 by the great development of the lagena which coils with two 

 or three turns to form the spiral cochlea. The monotremes 

 have the inner ear more on the sauropsidan plan. 



