=Causation.= Trichinosis is capable of attacking all mammifers without exception, from a man to a mouse; and most animals which can be made the subjects of experiment contract the disease in varying degrees. The intestinal form is seen in birds, but the muscles do not become infested by the embryos. Cold-blooded animals are proof against the disease. [Illustration: $1] After the ingestion of meat containing cysts of the parasite, the processes of gastric and intestinal digestion set the larvæ at liberty. These larvæ become sexual at the end of four to five days, and the females, which are usually twice as numerous as the males, begin laying eggs from the sixth day, continuing for a month to six weeks. Each female lays approximately from 10,000 to 15,000 eggs. The embryos perforate the intestinal walls, pass into the circulation, and are hurried into all parts of the system. This period of infestation constitutes the first phase of the disease. Askanazy, in 1896, suggested that it was not the embryos which perforated the intestinal walls and thus reached the blood-vessels, but the fertilised female trichinæ themselves, which entered the terminal chyle vessels and laid their eggs directly within them. This observation is of great interest, for it contradicts the view held by Leuckart and proves that treatment is useless even in the first phase. The males are about ¹⁄₁₆ inch in length, the females ⅛ inch to ⁵⁄₃₂ inch, and are ovoviviparous. =Symptoms.= The symptoms lack precise character, even when the disease is known to be developing, and moreover they have only been carefully observed in experimental cases. As soon as the laying period begins, signs of intestinal disturbance may be observed, possibly due to embryos perforating the intestinal walls (if we accept Leuckart’s view), or, according to Askanazy, to adult females penetrating the chyle vessels and disturbing intestinal absorption. These symptoms are only appreciable in cases of “massive” infestation. If slight, the disturbance passes unperceived. In severe cases the symptoms consist of diarrhœa, loss of appetite, grinding of the teeth, abdominal pain in the form of dull colic, and sometimes irritation of the peritoneum. The embryos carried by the circulation then escape into the tissues and, like the cysticerci, become encysted, preferably in the muscles, in the interfascicular connective tissue towards the ends of the bundles. Each (asexual) parasite plays the part of a foreign body, causing infiltration of serum and exudation of leucocytes in its neighbourhood, and soon becoming encysted in the interior of a little ovoid space surrounded by a fibro-fatty wall. Fat granules accumulate at each end of the cyst. [Illustration: $1] [Illustration: $1] [Illustration: $1]