DIGESTIVE SYSTEM 117 



4. Irritation of mucous membrane due to lip attachment result- 

 ing in inflammation, necrosis, and ulceration. 



5. Liberation of chemical substances that produce nervous irri- 

 tation or hematoiysis. 



The following parasites infest the intestine of swine: 



1. Ascaris suum. 



2. Esophagostoma dentatum. 



3. Trichocephalus crenatus. 



4. Trichinella spiralis. 



5. Gigantorhynehus hirudinaceus. 



Ascaris Suum 



Asearids are found in practically all hogs. They are not blood- 

 suckers in the ordinary sense, but may obtain nutrition direct from 

 lymph as well as by absorption through the external integument. 

 The impregnated female discharges her ova into the intestine and 

 they pass out in the feces. If conditions are favorable, the ova 

 hatch into embryos which become encysted. ^Mien su.h encysted 

 embryos are ingested the digestive juices dissolve their enveloping 

 membrane and the embryos are thus liberated. Some of the liberated 

 embryos pass directly through the walls of the digestive tube, others 

 are entangled in the content of the digestive tube and eliminated 

 with the feces and perish. Some of the embryos that pass through 

 the walls of the digestive tube gain entrance to blood capillaries and 

 are ultimately carried to various parts of the body by means of 

 the circulation. The major portion of these embryos probably perish. 

 Some of the embryos that have passed through the wall of the 

 digestive tube invade the tissues and those that finally reach the 

 air cells of the lung and bronchioles develop into larvae. After 

 the larvae are fully developed they pass out of the lung by way of 

 the air tubes and when they reach the pharynx some of them are 

 swallowed and upon reaching the intestine develop into adults. Thus 

 the embryonal and larval stage produce disturbances of the lung 

 and other organs which will be discussed in the chapters pertaining 

 to those organs. 



It has been demonstrated that the embryonal and larval stage of 

 the Ascaris suum may occur in rats and possibly also in other animals. 



Lesions. — These parasites are round, and pointed at either end; 

 they are white or pinkish-white in color and vary from six inches 

 to twelve inches in length. Usually they are found in the small 



