THE MICROSCOPE. 



points in their minute structure in which they differ ; and 

 one of these points is in the difference in size of their 

 bone-cells. To determine accurately, therefore, between 

 the two, we must, if the section be a transverse one, also 

 note the comparative sizes of the Haversian canals, and the 

 tortuosity of their course; for the diameter of the canal 

 bears a certain proportion to the size of the bone-cells, and 

 after some little practice the eye will readily detect the 

 difference." 



FISH. 



It has often been observed, that we are surrounded by 

 wonders which we do not notice because they are of daily 

 occurrence, but which excite the greatest surprise when they 

 are pointed out to us. The truth of this observation is 

 forcibly exemplified as regards fish. We see them every 

 day exposed for sale on stalls, and we eat them frequently 

 at our tables, without once considering by what a curious 

 and delicate organisation these creatures are enabled to 

 see and breathe in an element that carries death to us and 

 to quadrupeds. The sight of fishes appears to be remark- 

 ably strong, as it is by sight chiefly that they discover 

 their prey. Hence a fish is easily deceived by an artificial 

 fly, or the imitation of a frog or other small aquatic or 

 amphibious animal ; which, if it were guided by the smell, 

 or any other sense than the sight, could not happen. The 

 mode in which fishes breathe is, however, the most curious. 

 They have no lungs ; but, instead of them, they have gills, 

 carefully covered with a lid and a flap, both of which the 

 fish can open or keep closed at pleasure. The gills are 

 composed of arches bordered by a kind of fringe, which, 

 examined through the microscope, is seen to be a velvet- 

 like membranaceous covering ; and over this numberless 

 wonderfully minute blood-vessels are spread out like a deli- 

 cate network. There are commonly four of these fringed 

 arches, which are movable, and allow the currents of water 

 driven down by the action of the mouth to flow freely 

 through them, so as to lave every fibril. It is absolutely 

 necessary that this should be the case, since the gills 

 lose their power of acting as soon as they become dry; 

 hence a fish cannot live long after it is taken out of the 



