Text book of zoology / by J.E.V. Boas ; translated by J.W. Kirkaldy and E.C. Pollard.

  • Boas, J. E. V. (Johan Erik Vesti), 1855-1935.
Date:
1896
Catalogue details

Licence: Public Domain Mark

Credit: Text book of zoology / by J.E.V. Boas ; translated by J.W. Kirkaldy and E.C. Pollard. Source: Wellcome Collection.

    503/582 (page 481)
    oyer the outer surface of the eyeball when this is withdrawn into the orbit and covers it partially or entirely.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 scl erotic 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 tapetum {e.g., in the Horse, Ruminants, Carnivora). The form of the pupil varies, it is either circular {e.g., in Man,), or a perpendicular (Cat, Fox), or horizontal slit (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 Reptiles there is a fenestra ovalis and a fenestra rotunda. The single ear- Fig. 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 ampulla, b semi-circular canal (only one is represented) c cochlea; sa sacculus, u utriculus (together forming the vesti- bule) ; round the labyrinth the cavum perilymphaticum, black in the figure. h bones of the skull, h malleus, am incus, s stapes, t tympanic cavity, r fenestra rotunda, e Eustachian tube, tr 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 cartilaginous. t According to another interpretation the malleus is homologous with the quadrate of Reptiles, whilst the incus is to be regarded as representing the outer portion of the reptilian columella auris; others again regard the incus as corresponding to the quadrate, the malleus to the upper posterior bone of the mandible of Reptiles {articular e). I l