238 E VOL UTION AND DISEASE. 



tumours in fish, frogs, birds, snakes, marsupials, rodents, 

 carnivora, quadrumana, and ruminants. 



When sarcomata grow from bone, especially from 

 the interior of a bone, they usually possess large num- 

 bers of giant-cells. When originating in pigmented 

 spots such as the black or pigment coat of the eye, or 

 the pigment layer of the skin, they are of a deep black 

 colour, and named in consequence melanotic. Grey 

 horses are especially liable to this form of tumour, yet 

 we have no reason to believe that the coloured races of 

 mankind are more or even so prone to them as 

 Europeans. 



Sarcomata do not always remain localized in this 

 way. After the tumour has been growing for a time, 

 other nodules make their appearance in different parts 

 of the body, and not infrequently the secondary forma- 

 tions are larger than the original tumour. These facts, 

 and the general effects of such tumours, would alone 

 cause us to suspect some parasitic agent, and what is of 

 utmost importance, the early removal of the primary 

 tumour occasionally prevents general infection. 



In the chapter on Inflammation, the relation of the 

 leucocytes to bacterial invasion was described. Let us 

 ascertain how it will elucidate the nature of sarcomata. 

 The classification of these tumours is founded on the 

 structural characters displayed by thin sections of the 

 dead tumours under the microscope ; they are then 

 described as round-cell tumours, spindle-celled^ melanotic 

 or giant-celled. The appearance of a section of a round- 

 celled sarcoma is exhibited in fig. 121. 



When fluid portions of such tumours are examined, 



