412 THE POPULAR SCIENCE MONTHLY. 



marsupial bone, and thence stretches itself over the inner or deep sur- 

 face of the adjacent mammary gland or " breast," which is situated 

 low down, and not in the breast at all. 



The kangaroo's teeth consist of three on each side in the front of 

 the mouth, and one on each side below. These eight teeth are what 

 are called incisors. At the back of the mouth there are live grinding- 

 teeth on each side above and five below, and between the upper grind- 

 ers and incisors another pointed tooth, called a canine, may or may 

 not be interposed. Such a set of teeth is indicated by the following 

 formula, where I stands for incisors, C for canines, and M for grinding- 

 teeth or " molars." The number above each line indicates the teeth 

 of each denomination which exist on one side of the upper jaw, and 

 the lower number those of the lower jaw : 



3 15 9 8 



I _ C - or - M - = - or - 



10 5 6 6 



The total number of incisor teeth of both sides of each jaw may there- 

 fore be expressed thus : 1 1. 



Such is the general structure of an adult kangaroo. At birth it is 

 strangely different from what it ultimately becomes. 



It is customary to speak of the human infant as exceptionally 

 helpless at birth and after it, but it is at once capable of vigorous suck- 

 ing, and very early learns to seek the nipple. The great kangaroo, 

 standing some six feet high, is at birth scarcely more than an inch 

 long, with delicate naked skin, and looking like part of an earthworm. 

 But, in such feeble and imperfectly developed condition, the young- 

 kangaroo cannot actively suck. The mother therefore places it upon 

 one of her long and slender nipples (the end of which is somewhat 

 swollen), this nipple entering its mouth, and the little creature remain- 

 ing attached to it. The mother then, by means of the cremaster mus- 

 cle (before spoken of), squeezes her own milk gland, and so injects 

 milk into the young, which would thus be infallibly choked but for a 

 noticeable pecixliarity of its structure, admirably adapted to the cir- 

 cumstances of the case. 



In almost all beasts, and in man also, the air-passage or w^indpipe 

 (which admits air to and from the lungs) opens into the floor of the 

 mouth, behind the tongue and in front of the ojDening of the gullet. 

 Each particle of food, then, as it passes to the gullet, passes over the 

 entrance to the windpipe, but is prevented from falling into it (and so 

 causing death by choking) by the action of a small cartilaginous shield 

 (the epiglottis). This shield, which ordinarily stands up in front of 

 the opening into the windpipe, bends back and comes over that open- 

 ing just when food is passing, and so, at the right moment, almost 

 always prevents food from " going the wrong way." But, in the young 

 kangaroo, the milk being introduced, not by any voluntary act of tho 



