46 OF THE VITAL AND N f ufually inquired into, nor indeed eafily anfwered ; although the fécond feems to admit of a much eaher folution than the fir ft. If all the parts and organs exift in miniature in the animalcule in femine, it will fcarce be difpuced, that while it fwims in this liquor, the fluids are propelled through its veflels by the a&ion of its heart, and circulate in the fame manner as in the fœtus in utero. If the heart does not exift in the animalcule, but is formed after concep¬ tion, then the beginning of its motion mu ft be later. But, be this as it will, we know that in impregnated eggs, the animalcule lies in a death-like ftate, refembling that of many infe&s, and fome larger animals in winter ; and that its heart remains at reft, till by the heat of incubation it is roufed into action. After the motions of the heart of the chick become vifible, they may be rendered more lively or languid by a greater or lefs degree of warmth ; nay be made entirely to ceafe by cold, and be as quickly renewed again by heat *. Hence it follows, that though it be not certain when the heart begins firft to move in nafcent animals, yet the caufe which fets it firft a-going, and recommences its motions after being flop¬ ped, is heat, which, by rarifying and agitating with an inteftine motion the particles of the fluids, enables them, by their Jiimulus, to excite the fibres of the heart into contra&ion ; at the fame time that it increafes the fenfibility of the nerves, which are benumbed by too great a degree of cold. SECT. V. Of the motions of the alimentary canal, and bladder of urine. HAVING thus accounted for the alternate contra&ion and re¬ laxation of the heart ; we come next to inquire into the caufe of the other involuntary motions ; and {hall begin with thofe of the alimentary canal. In deglutition, the contra&ion of the mufcles which draw up the larynx # Harvey de generation, animal, exercitat. 17.