672 



THE MICROSCOPE. 



Fig. 304. — Pigment cells 

 from the eye. 



races. In the eye there is pigment, and it affords a good 

 example of nucleated cells, in which are contained the 

 pigment particles, fig. 304. These are placed there for an 



optical purpose, that of absorbing 

 the rays of light. In the peculiar 

 Jlllig* colouration found in the eyes of some 

 |?%S^§v* animals, called Tapetum lueidum, the 

 * colour is not owing to the pigment 

 f| particles, but to the interference of 

 the light : it is reflected from it, as 

 in mother-of-pearl, coloured feathers, 

 scales of fishes, &c. The colour of 

 the skin is owing to the granular 

 contents of the pigment cells ; these 

 are like ordinary elementary gra- 

 nules, with the addition of colour; 

 and this latter may be removed by 

 the action of chlorine. 



The Nails are appendages to the 

 epidermis, and present a mould of 

 the cutis beneath ; from the cutis 

 the materials are furnished for the 

 formation and growth of the nail. 

 Like the epidermis, the nail is 

 stratified, the markings are parallel 

 to the surface, and the appearance 

 is produced by the coalescence of the 

 cells and their lying over each other. 

 See Plate VII., No. 149, toe of mouse. 

 The stratified arrangement when a 

 section is examined by polarised 

 light, presents the appearance seen 

 in the processes of the cat's tongue, 

 Plate VIII. No. 174. 



Hairs. — The form and structure 

 of hairs differ much ; some are cy- 

 lindrical, others flattened. A hair 

 is divided into a body or shaft, and 

 a root which is in the skin (fig. 

 Fig. 305.— A single Hair, 305.) The shaft is again divided 



seen near its bulb. ^tfo twQ parts . fljg external is 



termed the cortical portion, and the internal the medul- 



