Note the very compact _skeleton_ of the mouse. Note the closely sutured _skull_. How many _cervical_ or _neck vertebræ_ are there? The _ribs_ are attached to the _thoracic vertebræ_. How many pairs of ribs? The bony thorax supports the _shoulder-girdle_ and bones of the fore legs. The thorax is followed by a series of ribless vertebræ, the _lumbar vertebræ_, which in the posterior region of the body fuse with the _pelvic girdle_ supporting the hind limbs. The body vertebræ are succeeded by the very much smaller _caudal vertebræ_. Compare the skeleton of the mouse with that of the bird; also with that of the toad. For directions for a detailed study of the skeleton see in Parker's "Zootomy" an account of the skeleton of the rabbit, pp. 263-286. TECHNICAL NOTE.--For the study of the eye (fig. 150) the teacher should obtain the eye of some large mammal, as the ox or sheep, with which to make a class demonstration. The eye of a rabbit or cat can of course be used. For an account of the vertebrate eye see Parker and Haswell's "Text-book of Zoology," Vol. II. pp. 103-107. For a study of the ear use a bird or mammal, and see pp. 107-110 of the same book. [Illustration: FIG. 150.--Diagram of vertebrate eye; _c_, choroid; _i_, iris; _l_, lens; _n_, optic nerve; _r_, retina; _s_, sclerotic. (From Kingsley.)] =Life-history and habits.=--The house-mouse is not a native of North America, but was introduced into this country from Europe, to which, in turn, it came from Asia, its original habitat. The mouse came to this country in the vessels of early explorers. Similarly the brown and black rats, now so abundant all over North America, and members of the same genus as the mouse, were introduced from Europe. Accompanying man in his travels the mouse has spread from Asia until it is now to be found over the whole world. The habits of mice are well known; their fondness for living in our homes and outbuildings makes them familiar acquaintances. Their food is varied; they seem to thrive best, however, on a vegetable diet. Grains and nuts are favorite foods. The house-cat is their greatest enemy, but man takes advantage of their instinct to go into holes by constructing traps with funnel or tunnel entrances which, baited with cheese or other favorite food, are fatally attractive. In climbing, mice are aided by the tail. Their strong hind legs enable them to stand erect, and even to take several steps in this posture. They can swim readily, although naturally they rarely take to water. Their special senses are keen, the senses of hearing and taste being unusually well developed. Their "singing," which has been the subject of much discussion, seems to be actually a voluntary and normal performance which, however, hardly deserves to be called singing, but rather a slightly varied peeping or whistling.