674 MAMMALIA. 



of the milk premolars. Into the toothless gap or diastema 

 between the front and back teeth, the hairy skin of the lips 

 projects into the mouth. This generally occurs in Rodents, 

 and is said to prevent the inedible substances which they 

 gnaw from passing backwards to the gullet. On the sides of 

 the snout, and about the eyes, there are tactile hairs or 

 vibrissse. 



The plump trunk is separated from the head by a short 

 neck. The tail is very short, but in the scampering wild 

 rabbit it is conspicuous as a white tuft, which some natural- 

 ists interpret as a directive signal. Beneath the base of 

 the tail the food canal ends, and beside the anus are the 

 openings of the perineal glands, whose secretion has a char- 

 acteristic odour. In front of the anus is the urogenital 

 aperture, in the male at the end of an ensheathed penis, 

 in the female a slit or vulva, with an anterior process or 

 clitoris the homologue of the penis. Beside the penis in 

 the male lie the scrotal sacs, into which the testes descend 

 when the rabbit becomes sexually mature. Along the 

 ventral surface of the thorax and abdomen in the female 

 there are four or five pairs of small teats or mammae. 



The limbs have clawed digits, five on the fore-feet, four 

 on the hind-feet ; they are very hairy. 



Skin and muscles.- -The skin is thickly covered with 

 hair, and has the usual sebaceous and sudorific glands, 

 besides special glands, such as the perineal glands beside 

 the anus, the glands of the eyelids, the lachrymal glands, 

 and the mammary glands developed in the females. 

 Between the skin and the subjacent muscles there is a layer 

 of fatty tissue, known as the panniculus adiposus ; it is present 

 in all Mammals except the common hare, and forms the 

 blubber of whales and seals. Beneath the skin is a thin 

 sheet of muscle (the panniculus carnosus), by means of 

 which the skin can be twitched, as in horses, etc., and when 

 this is removed with the skin, many of the muscles of 

 head and neck, limbs and trunk, are disclosed (see Parker's 

 "Zootomy"). 



Skeleton.- -The bones, like those of other Vertebrates, 

 are developed either as replacements of pre-existent cartil- 

 ages, or independent of any such preformations, but in all 

 cases through the agency of active periosteal membranes. 



