438 BENIGN TUMOURS. 



are also certain cysts containing milk that are formed of enlarged 

 lactiferous tubes, such as the ovarian cysts, formed by the over- 

 grown Graafian vesicles; and such appear to be certain cysts 

 formed of dilated portions of blood-vessels shut off from the main 

 streams. 



Sd. Many, and perhaps the great majority of cysts, such as 

 those of the kidney, the choroid plexuses, the chorion, and the 

 thyroid gland, are formed by the enormous growth of new ele- 

 mentary structures, having the character of cells or nuclei, which 

 pursue a morbid course from their origin, or from a very early 

 period of their development. 



Simple or barren cysts, as they occur in veterinary practice, 

 may be divided into gaseous, serous, synovial, and mucous. 



Gaseous cysts have been observed by Hunter on the intes- 

 tines of pigs. I have frequently seen them on the intestines 

 of dogs. A cyst of this kind often forms on the hind quarters 

 of horses, in the region of the trochanter major of the femur, 

 which does not seem to contain any fluid when cut into, and 

 upon dissection presents a mere cavity, lined by a glistening grey 

 membrane. It can be distinctly observed in the living animal 

 Can the term wind-gall, as applied to distended synovial bursse, 

 have arisen from the fact that this cyst seemingly contained 

 nothing but air ? 



Serous cysts arise in two ways: — 1st. From the effects of 

 pressure ; and 2d., without evident cause. To the first belong 

 capped hock, capped elbow, and other so-called serous abscesses. 



Serenes abscesses are soft tumours, formed by an effusion of serum 

 into the areolae of the connective tissue of the part, which be- 

 comes condensed, and marks out the limits of the cyst. Within 

 the cyst the fibrous trabeculse, or bands of connective tissue, are 

 gradually absorbed, and give place to the effusion, until at last 

 a cavity is formed, which is filled with serum, and lined by a 

 more or less perfect epithelium. In some serous abscesses, more 

 particularly if caused by great pressure or friction, inflammatory 

 products are formed, which become consolidated; or a per- 

 fectly formed cyst may inflame, and become converted into a 

 solid tumour. In others again, the areolar structure is not so 

 completely removed, and the serosity will be found in various 

 alveoli or spaces, giving to the tumour a honeycomb appearance 

 when cut into. 



