74 GENERAL ANATOMICAL CHARACTERS 
—has a great tendency to assume the columnar form of the 
corresponding bone in Sauropsida, its two rami entirely or partially 
coalescing.1 The tympanic membrane (drum of the ear) forms the 
outer wall of the cavity. In the fcetal state it is level with the 
external surface of the skull, and remains so permanently in a few 
mammals, as the American Monkeys ; but commonly, by the growth 
of the squamosal bone, it becomes deeply buried at the bottom of a 
bony tube (meatus auditorus eaternus), which is continued to the sur- 
face of the skin in a fibrous or fibro-cartilaginous form. In Whales, 
owing to the thickness of the subcutaneous adipose tissue, this 
meatus is of great length, and is also extremely narrow. In most 
aquatic and burrowing animals it opens upon the surface by a simple 
aperture, but in the large majority of the class there is a projecting 
fold of skin, strengthened by fibro-cartilages, called the pinna, 
auricle, or “external ear,” of very variable size and shape, generally 
movably articulated on the skull, and provided with muscles to 
vary its position ; this pinna helping to collect and direct the vibra- 
tions of sound into the meatus. 
VII. REPRODUCTIVE ORGANS. 
Testes.—In the male the testes retain nearly their primitive or 
internal position throughout life in the Monotremata, Sirenia, 
Cetacea, most Edentata, Hyracoidea, Proboscidea, and Seals, 
but in other groups they either periodically (as in Rodentia, 
Insectivora, and Chiroptera) or permanently pass out of the 
abdominal cavity through the inguinal canal, forming a projection 
beneath the skin of the perineum, or becoming suspended in a 
distinct pouch of integument called the scrotum. All the Marsupials 
have a pedunculated scrotum, the position of which differs from 
that of other mammals, being in front of, instead of behind, the 
preputial orifice. As regards the presence, absence, or comparative 
size and number of the accessory generative glands—prostate, vesi- 
cular, and Cowper’s glands, as they are called—there is much 
variation in different groups of mammals. 
Penis.—The penis is almost always completely developed, 
consisting of two corpora cavernosa attached to the ischial bones, 
and of a median corpus spongiosum enclosing the urethra, and 
forming the glans at the distal portion of the organ. In Marsupials, 
Monotremes, and the Sloths and Anteaters, the corpora cavernosa 
are not attached directly to the ischia, and in the last-named the 
penis is otherwise of a very rudimentary character, the corpus 
1 The modifications of these bones are fully described by A, Doran, ‘‘ Morpho- 
logy of the Mammalian Ossicula auditus,” Trans. Linn. Soc. ser. 2, vol. i. pp. 
371-497, pl. lviii.-lxiv. (1878). 
