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 foetal 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 externus), 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. L pp. 

 371-497, pi. lviii.-lxiv. (1878). 



