HORNS, HOOFS, CLAWS 



1033 



in which the anterior and posterior wings of certain hymenopterous 

 insects are locked together. Feathers or portions of feathers of 

 1 lirds distinguished by the splendour of their plumage are very good 

 objects for low magnifying powers when illuminated on an opaque 

 ground ; but care must be taken that the light falls upon them at 

 the angle necessary to produce their most brilliant reflection into 

 the axis of the [microscope; since feathers which exhibit the most 

 splendid metallic lustre to Vin observer at one point may seem very 

 dull to the eye of another in a different position. The small feathers 

 of humming-birds, portions of the feathers of the peacock, and 

 others of a like kind, are well worthy of examination ; and the 

 scientific microscopist, who is but little attracted by mere gorgeous- 

 ness, may well apply himself to the discovery of the peculiar 

 structure which imparts to these objects their most rema.rka.ble 

 character. 1 



Sections of horns, hoofs, claws, and other like modifications of 

 epidermic structure which can be easily made by the microtome, 

 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 polarised light produces more remarkable effects, or which 

 display a more beautiful variety of colours when a plate of selenite is 

 placed behind them and the analysing prism is made to rotate. A 

 curious modification of the 

 ordinary structure of horn is 

 presented 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 independ- 

 ent papilla? (fig. 765). When 

 transverse sections of these 

 cylinders are viewed by polar- 

 ised light, each of them is 

 seen to be marked by a cross. PIG. 765. Transverse section of horn of 

 somewhat resembling that of rhinoceros viewed by polarised light. 



starch-grains: and the light 



and shadow of this cross are replaced by contrasted colours when 

 the selenite plate is interposed. The substance commonly but erro- 

 neously termed irhalebone, 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 

 rhinoceros-horn, and is similarly affected by polarised light. The 

 central portion of each of its component threads, like the medullary 



1 See R. S. Wray, ' On the Structure of the Barbs, Barbuk-s, and Barbicels of a 

 typical Pennaeeou* Feather,' in the Ibis for 1887, p. 4'2t>. 



