Glass 6. Mammalia. 



481 



over the outer surface of the eyeball when this is withdrawn into the 

 orbit and covers it partially or entirely. Sometimes {e.g., in Man) it 

 is rudimentary. 



The sclerotic consists of connective tissue without cartilage* or bone ; 

 in some Mammalia, especially in the Whales, it is very thick. In the choroid 

 coat there is frequently a peculiar greenish, blue, or whitish, shimmering 

 membrane varying somewhat in structure, the tape turn [e.g., in the Horse, 

 Rumiaants, Oarnivora). The form of the pupil varies, it is either circular 

 {e.g., in Man,), or a perpendicular (Cat, Fox), or horizontal sht (Horse, 

 Ruminants). 



Auditory organ. The cochlea of Monotremes is 

 like that of Crocodiles and Birds ; in all other Mammalia, however, 

 it is much longer and is spirally coiled. As in Keptiles there is a 

 fenestra ovalis and a fenestra rotunda. The single ear- 



Pig. 392. Diagrammatic trans- 

 verse section of the head of a Mammal, 

 to show the relations of the auditory 

 organ ; (the labyrinth is drawn propor- 

 tionally much too large, etc.). a 

 ampuUa, 6 semi-circular canal (only one 

 is represented) c cochlea ; sa saoculus, 

 M utriculus (together forming the vesti- 

 bule) ; round the labyrinth the cavum 

 perilymphatioum, black in the figure, 

 fc bones of the skull, % malleus, am 

 incus, s stapes, t tympanic cavity, r 

 fenestra rotunda, e Eustachian tube, 

 ir tympanic membrane, g external 

 auditory meatus, o external ear. — 

 Orig. (with partial use of older 

 figures). 



bone of Reptiles is broken into three, the malleus, which is connected 

 with the tympanum, the incus," and the stapes, the terminal disc of 

 which closes the fenestra ovalis ;t in the Monotremes it consists of a 

 plate and a single shaft; which is usually broader and perforate in 

 other orders, so that the ossicle becomes like a stirrup. The presence 

 of an external meatus is characteristic of the Mammalia ; the 

 pit, at the base of which the tympanum is situated in Reptiles, has 

 become a long tube here ; the inner part is often ossified (a tubular 

 elongation of the tympanic bone), whilst the external portion is 

 supported by cartilages. The external auditory meatus is usually 

 surrounded by the pinna, a large fold of skin varying in form 

 and containing a considerable amount of elastic cartilage. 



* In the Monotremes, the sclerotic is partly oartUaginous. 



t Aooordiug to another interpretation the malleus is homologous with the quadrate 

 of Eeptiles, whilst the incus is to be regarded as representing the outer portion of the 

 reptilian columella amis ; others again regard the incus as corresponding to the 

 quadrate, the malleus to the upper posterior bone of the mandible of Eeptiles 

 {wrticMlare). 



I I 



