356 



THE MICROSCOPE. 



fig. 168. 



it takes the same course. The ciliary motion may be seen in the kid- 

 ney of the frog or newt ; the cilia in the latter continue in active mo- 

 tion for some minutes after the animal is dead. A thin section must 

 be made, with a very sharp knife and care taken to disturb the structure 

 as little as possible. The section should be moistened with a little of the 

 serum of the animal, and examined in a glass cell, covered with thin glass. 

 Pigment. — Pigment granules are found in greater or less quantities 

 in the skin and bodies of white and dark races. 

 In the eye there is pigment, and it affords a 

 good example of nucleated cells, in which are 

 contained the pigment particles, fig. 168. These 

 are placed there for an optical purpose, that of 

 absorbing the rays of light. In the peculiar co- 

 louration found in the eyes of some animals, called 

 Tapetum lucidum, the colour is not owing to the 

 pigment 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 granulous contents of the pigment cells ; these are like 

 ordinary elementary granules, 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 ap- 

 pearance is produced by the coalescence of the cells and their lying 

 over each other. This arrangement causes an irridescent appearance of 

 the section, when viewed with polarised light under the microscope. 



Hairs.- — -The form of these differs in different parts of the body : 

 some are cylindrical, others flattened. A hair is. divided into a body 

 or shaft, and a root which is in the skin (Plate XIV., No. 2). The 

 shaft is again divided into two parts : the external is termed the cor- 

 tical portion, and the internal the medullary portion ; the latter does 

 not usually exist in the whole length of the shaft. The cortical part 

 consists of fibres, arranged parallel to each other r. besides these there 

 are, on the exterior, minute scales, like an epithelium ; which are 

 arranged like the tiles of a house, and produce the appearance of trans- 

 verse markings. The fibres gradually expand out, forming a wall to 

 the bulb enclosed in its capsule. The development of a hair com- 

 mences at the bottom of the follicle, and by the aggregation of succes- 

 sive cytoblasts or new cells is gradually protruded from the follicle, 



