INTRODUCTION 



large and fullj and several of the Carnivora have a very keen sight, 

 though not so acute perhaps as that of many raptorial birds. The 

 eyelids in most are double as in Man, but in some of the lower forms 

 a remnant of the nyctitating membrane is also met with. A lach- 

 rymal apparatus exists in most animals except the Cetacea. 



The sense of hearing attains its greatest perfection in Mammals, 

 and an external ear is present in most, generally mobile, to assist in 

 conveying the sound from every direction, and concentrating it. The 

 internal ear is imbedded in the petrous portion of the temporal 

 bone, and is remarkable for the development and complexity of the 

 cochlea. The tympanic cavity is large, and communicates with the 

 pharynx by the eustachian tube. The communication between the 

 membrane of the tympanum and the inner ear, v.'hich in the other 

 vertebrates takes place by one ossicle, here consists of four. 



Owing to the fleshy nature and mobility of the tongue, the sense 

 of taste is much greater than in any other animals. 



The leading peculiarity of the class of Mammals, viz., the nourish- 

 ing of the new bom young with milk has been already alluded to. 

 The milk is secreted by the mammary glands, and these vary in 

 number and position, being most numerous in the more prolific 

 races. In most animals they are ventral, inguinal in many, and 

 pectoral in a few. They vary in number from two to twelve. The 

 yelk of the ovum which suffices for the nourishment of the young of 

 Reptiles and Fishes, only affords a very small amount of nutriment 

 to the Mammalian ovum, when expelled from the ovary, and the 

 foetus is supported in the uterus by the blood of the mother, con- 

 veyed by means of a vascular connection, called the placenta, the 

 shape and situation of which, within the uterus, varies considerably. 

 In one tribe of animals, however, the Marsupials, the young are ex- 

 pelled from the uterus at a very early period of development, and 

 receive no nourishment beyond that derived from the yelk, no pla- 

 centa being formed. 



The young of several animals are born blind, others with their 

 eyes open. Most are helpless at first, whUst a few are able to run 

 about very shortly after birth ; some attain their full size in a few 

 months, others take several years. 



