CHAPTER XL 



TUMOURS AND CANCERS. 



TUMOURS are very interesting to the evolutionist, and in 

 order to obtain a clear notion of them it will be necessary 

 to classify the various " swellings " to which the term is 

 applied, and this is more essential as even medical men 

 use the word in a very indefinite sense. The term 

 " tumour," which literally signifies " a swelling," com- 

 prises Cysts, Infective Tumours, Sarcomata, Neoplasms, 

 and Cancers. Each requires separate consideration. 



CYSTS. A cyst is a tumour containing fluid or semi- 

 fluid contents resulting from the dilatation of a pre-existing 

 cavity. 



Tumours conforming to this definition arise in different 

 ways. For instance, many organs such as the kidneys, 

 liver, salivary glands, and the like, are furnished with a 

 duct, or series of ducts, whereby the fluid secreted by 

 them is conveniently discharged. If from any cause the 

 fluid be prevented from escaping after it has been 

 secreted, it will distend the ducts until they become 

 dilated into large reservoirs, or retention-cysts, as they 

 are termed. This is well illustrated in the case of 

 the kidneys represented in fig. 116. The drawing 

 shows the kidneys with their ducts (ureters) and the 

 urinary bladder of a terrier. The bladder contains two 



