7 HOW TO WORK 



of fishes, sections of bone, teeth, horn, hoofs, claws, nails, specimens 

 of various kinds of hair, are examples of objects derived from the 

 animal kingdom which may be examined in this manner and 

 mounted permanently if desired. 



If the observer desires merely to ascertain how a structure looks 

 when examined in a highly refractive medium like balsam, he may 

 use turpentine, which can afterwards be dissipated by evaporation. 



Oil is an advantageous highly refracting medium for examining 

 certain structures in. The entozoa which may often be obtained 

 from the oily sebaceous matter squeezed from the follicles of the skin 

 of the nose or scalp, should be immersed in oil. They can generally 

 be found in the wax from the ear. 



Some tissues may be made to present different appearances 

 although mounted in the same medium. Thus bone exhibits very 

 different characters when immersed in Canada balsam, according to 

 the manner in which it is mounted. In every part of one specimen, 

 small black spots of irregular shape may be seen. From these a 

 number of minute dark lines radiate, and inosculate pretty freely 

 with corresponding lines from other spots. In another preparation 

 the entire section may appear perfectly clear, and its structure nearly 

 uniform everywhere. The first appearance is produced when a section 

 is mounted in old viscid balsam ; the second when it is immersed 

 in fluid balsam, after having been previously wetted with turpentine. 



The cause of these differences is interesting and worthy of attentive 

 study. The little black spots (lacunae) and dark lines (canaliculi) 

 were formerly considered to be small solid bodies, and the spots were 

 improperly termed bone corpuscles. They consist in truth of little 

 cavities or spaces in the bony tissue, and contain air. In the second 

 specimen the highly refracting oil of turpentine passed up the 

 canaliculi and into the lacunae driving out the air, thus rendering 

 the tubes and spaces invisible. The lacunae containedjn the fresh 

 bone, masses of germinal matter (nuclei), but when the bone had 

 become dry, the moist material dried up, and air rushed into the 

 lacunae and canaliculi to supply its place. The great difference 

 between the refracting power of the air contained in these little 

 cavities, and the surrounding osseous tissue gives rise to their dark 

 appearance. The above remarks upon the structure of bone apply 

 only to the dead and dried tissue. 



OF PREPARING TISSUES FOR MICROSCOPICAL EXAMINATION OF DIS- 

 SECTING AND CUTTING THIN SECTIONS OF TISSUES. 



144. Of Making Minute Dissections. Minute dissections are usually 

 carried on under the surface of fluid with the aid of small scissars, 



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