598 



VERTEBKATED ANIMALS. 



Microscopist who is but little attracted by mere gorgeousness, 

 may well apply himself to the discovery of the peculiar struc- 

 ture, which imparts to these objects their most remarkable cha- 

 racter. 



413. Sections of Horns, Hoofs, Claivs, and other like modifica- 

 tions of Epidermic structure, which may be made by the 

 Section instrument ( 107), the substance to be cut having been 

 softened, if necessary, by soaking in warm water, do not in 

 general afford any very interesting features, when viewed in the 

 ordinary mode; but there are no objects on which Polarized 

 light produces more remarkable effects, or which display a more 

 beautiful variety of colors, when a plate of selenite is placed be- 

 hind them, and the analyzing prism is made to rotate. A 

 curious modification of the ordinary structure of horn, is pre- 

 sented in the appendage borne by the Rhinoceros upon its snout, 

 which in many points resembles a bundle of hairs, its substance 

 being arranged in minute cylinders around a number of separate 

 centres, which have probably been formed by independent 

 papillae (Fig. 313). When transverse sections of these cylinders 

 are viewed by polarized light, each of them is seen to be marked 

 by a cross, somewhat resembling that of starch-grains ; and the 

 lights and shadows of this cross are replaced by contrasted colors, 

 when the selenite-plate is interposed. The substance commonly 

 but erroneously termed Whalebone, which is formed from the 

 surface of the membrane that lines the mouth of the whale, and 

 has no relation* to its true bony skeleton, is almost identical in 

 structure with Khinoceros-horn, and is similarly affected by 

 polarized light. The central portion of each of its component 

 fibres, like the medullary substance of hairs, contains cells that 

 have been so little altered as to be easily recognized ; and the 



outer or cortical portion also 

 may be shown to have a like 

 structure, by macerating it in 

 a solution of potass, and then 

 in water. Sections of any of 

 the horny tissues are best 

 mounted in Canada balsam. 

 414. Blood. Carrying our 

 Microscopic survey, now, to 

 the elementary parts of which 

 those softer tissues are made 

 up, that are subservient to 

 the active life of the body, 

 rather than to its merely me- 

 chanical requirements, we 



Transverse Section of Horn of RUnoceros, viewed shall in the first place notice 



by Polarized Light. the isolated floating cells 



contained in the Blood, and 

 known as the " blood-corpuscles." These are of two kinds; the 



Fm. 313. 



m 



