26 THE AMERICAN MONTHLY [Febniary, 



of water is prevented. The tubes of the freezing-box are so arranged 

 as to prevent their revoking with the revohitions of the table. In B is 

 shown a sectional view of the freezing arrangement. 



When ether is used the little brass plug in front of the freezing-box 

 is removed, and the rubber tubing detached. 



In preparing to make sections, remove the freezing-box and in its 

 place substitute a cork which projects suitably, holding the object from 

 which sections are to be taken, embedded in wax or paraffine, at the 

 required angle to the blade of the knife. The cork is raised or lowered 

 by means of a finely cut screw-thread. 



The curved knife is about five inches in length and one inch in 

 breadth, ground flat on the under side, and held in position by a bind- 

 ing screw, after the fashion of several microtomes previously in use. A 

 straight knife maybe used if desired. 



Introduction to Elementary Biology. 



By henry L. OSBORN, 



HAMLINE, MINN. 



Part VII. — A Specific Life History Considered Biologically. — 

 The Frog. 



( Continued from p. /j. ) 



The tissues of motion in the frog having now been shown to be 

 groups of cells or cell derivates which have specialized the power of 

 movement, it is natural that we should enquire whether the other pro- 

 toplasmic powers have been specialized and other tissues formed with 

 the special functions of irritability, metabolism, and reproduction. And 

 we shall find that such has been the case. At present we shall look at 

 the function of irritability. If any of the protozoa are suddenly shocked 

 or stimulated by a fforce without their bodies they quickly withdraw 

 from that force and we call this a case of irritability, saying that the 

 animal is in some degree sensitive to its surroundings. This power to 

 know its surroundings is of the utmost importance to a living being for 

 it must plainly precede the power to adjust itself to the surroundings — 

 to utilize the useful, to shun the hurtful. Most plants lack the power 

 of motion and sensation or if they have them at all they are extremely 

 feeble, while on the other hand most animals show the presence of 

 both powers in a more or less considerable degree. We should then 

 expect a sensor}- organization of some kind in as high an animal as the 

 frog. 



If we decapitate a vigorous, healthy frog and suspend it by the front 

 end of the trunk allowing the legs to hang down, and then dip the tip 

 of the longest toe in a weak solution of acetic acid (i%) the muscles 

 of the hip and knee joints will gentlv flex those joints and the foot will 

 be slowly lifted from the acid. Such an action is known in physiology 

 as a reflex action. A moment's consideration shows us that we have 

 not done anything directly to the muscles whose shortening lifted the 

 legs, for they are several inches away from the seat of the irritation. 

 And yet the muscular action Was caused by the irritation. If we 

 examine the frog's leg we shall find white threads, some of which go 

 to the skin and others to the muscles. Tin-sc are the nerve trunks 

 or •• ner\es.'" Tlie\ arc cuinpo.scfl of luuncruus line threads, too fine 



