HIGH SCHOOL ZOOLOGY. 165 



ward into the mucous membrane of the complicated labyrinth, 

 through the perforated or sieve-like plates of the ethmoid. The 

 right and left nasal cavities are separated by the perpendicular 

 plate of the ethmoid, and by the cartilaginous i)artition 

 {Septum), which extends as far as the nostrils. Certain small 

 cartilages not present in the lower forms are developed in the 

 external nose for its suppoi't, and for the attachment of the 

 muscles which move the nosti'ils. An organ of Jacobson, such 

 as is well developed in the snakes, is present in the cat, but it 

 has only a cartilaginous capsule, and it opens into the mouth 

 on each side through the incisive foramina. In the far-back 

 position of the posterior nostrils, the cat and other mammals 

 resemble such reptiles as the turtles and crocodiles, but the 

 pterygoid bones do not take part in bounding the nostrils. 



10. Reference has been made above to the great develop- 

 ment of the tympanic cavity, but there are also other modifica- 

 tions of the auditory apparatus, wliich make the ear of the 

 mammal more efficient (Fig. 104). In the first place the outer ear, 

 or pinna, is formed with its supporting cartilages, which serve for 

 the attachment of the muscles which move it. The chain of 

 bones, also, which effects communication between the tympanic 

 membrane and the labyrinth, is more complex ; it contains tliree 

 elements, the malleus, incus and stapes, of which the 'first 

 is connected in the young with Meckel's cartilage while 

 the last, a stirrup-shaped bone, occupies the fenestra ovalis. 

 Zoologists at one time thought that from the connection of the 

 malleus with Meckel's cartilage, the former bone must be com- 

 parable to the quadrate of lower forms, but the view is gaining 

 gi'ound that the whole chain is comparable to the columella 

 auris of the lower foi-ms. 



With regard to the labyrinth, an important advance in struc- 

 ture, which we meet with in most mammals as compared with 

 the Srturopsida, is that the cochlea, instead of being a straight 

 11 



