GENERAL PATHOLOGIC CHANGES 239 



The following kinds of cysts are recognized: (1) Retention cysts, 

 which occur when the characteristic secretions of glands accumu- 

 late in the ducts of the glands. The cause of their formation is 

 obstruction to the outflow of the fluid due to stenosis of the 

 duct, foreign bodies, pressure, or other forms of mechanical 

 obstruction. Examples are kidney cysts and mucous gland 

 cysts. (2) Parasitic cysts, which are formed as a result of para- 

 sites imbedding themselves in the tissues. Tapeworm cysts in 

 the muscles and those due to worms lodging in the testicles are 

 examples. (3) Dermoid cysts, which are caused by wandering 

 cells from the skin or mucous membranes during embryonic 

 life. An example is the tooth follicle cyst found at the base of 

 the ears in horses. In these cases probing will reveal one or 

 more teeth, which should be removed by a surgical operation. 



PARALYSIS 



Paralysis is loss of nervous control over the muscles that 

 govern the various body activities. It is general when all the 

 body muscles are affected; local when the muscles of a certain 

 part are affected. The cause is interference with the motor 

 nerve supply. It may be due to injury to the nerves or to the 

 centers controlling the nerves. When the loss of control is 

 slight or incomplete, the term paresis is used to designate the 

 condition. 



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