30 VETERINARY PHYSIOLOGY 



(C) Pigment Cells. In various parts of the eye the connec- 

 tive tissue and other cells contain a black pigment Melanin. 

 The precise mode of origin of this pigment is not known. It 

 contains carbon, hydrogen, nitrogen, oxygen, and it may also 

 contain iron. It may be formed directly in the cells, or it 

 may be produced by the cells from the pigment of the blood. 

 Its function in the eye is to prevent the passage of light 

 through the tissues in which it is contained. 



The cells containing the pigment are branched, and in many 

 cases they possess the power of movement. This is specially 

 well seen in such cells in the skin of the frog, where contraction 

 and expansion may be easily studied under the microscope. By 

 these movements the skin, as a whole, is made lighter or darker 

 in colour. The movements of these cells are under the control 

 of the central nervous system. 



2. Lymph Tissue. One peculiar modification of fibrous tissue 

 is often described as a special tissue under the name of Lymph 

 Tissue. It is composed of a delicate network of white fibres, the 

 interstices of which communicate with lymphatic vessels, and 

 contain masses of simple protoplasmic cells, lymphocytes, often 

 in a state of active division. So numerous are these that it is 

 impossible to make out the network under the microscope 

 until they have been removed by washing. 



Lymph tissue is very widely distributed throughout the body, 

 and is of great importance in connection with nutrition. 



3. Cartilage. "While fibrous tissue is the great binding 

 medium of the body, support is afforded in fostal life and in 

 certain situations in adult life by cartilage. 



When cartilage is to be formed, the embryonic cells become 

 more or less oval, and secrete around them a clear pellucid 

 capsule. This may become hard, and persist through life, 

 as in the so-called parenchymatous cartilage of the mouse's 

 ear. 



(1) Hyaline Cartilage. Development, however, usually goes 

 further, and before the capsule has hardened, the cartilage cells 

 again divide, and each half forms a new capsule which expands 

 the original capsule of the mother cell, and thus increases the 

 amount of the formed material. This formed material has a 



