This worm is met with in the muscular tissue of man, and occurs there in immense numbers, producing the disease Trichinosis. We shall see afterwards that this is not the mature form of the worm, but it is in this form alone that, for the most part, it is accessible to us, and it will be convenient to begin with its description here.

The affected muscles, as seen with the naked eye, seem for the most part to be dusted throughout with fine white particles like sawdust. These are most abundant near the places where the muscular fibres are inserted into the tendons. As a rule the particles are most abundant in the muscles of the trunk, the diaphragm, the intercostal muscles, and those of the abdominal wall, but they may extend to all the voluntary muscles of the body, even the most distant ones of the hands and feet. It may be present in immense numbers, even in millions, in the same person.

On microscopic examination of the fine particles, they are found to consist each of an oval cyst with a tolerably thick wall (see Fig. 193), within which is a small worm coiled up in a spiral manner. The cyst has very often abundant calcareous particles in its wall, especially at the poles, and, if the case is an old one, the impregnation with lime may be so great as to hide the parasite unless the salt be first dissolved out with an acid. When an acid, such as dilute hydrochloric, is used, the lime dissolves with some evolution of gas, and the whole structure becomes very transparent. Sometimes the worm dies in its capsule, and in that case the wall thickens and the cyst collapses to some extent on the remains of the worm, which itself often becomes infiltrated with lime (see lowest specimen in figure).

The parasite in.man is derived from the pig, in whose muscles the embryoes occur in the same fashion as in man. If, now, a piece of muscle containing them in their living state - that is to say, not killed by cooking the meat - be eaten, they undergo further development in the intestinal canal. The capsule is dissolved by the gastric juice, and the embryo set free. In the muscle, the embryo, if uncoiled, would measure about the twenty-fifth of an inch in length; but now it grows rapidly, and in the course of two and a half days it reaches the adult form, when the female is about one-eighth of an inch in length, and the male slightly less. The male possesses a testicle consisting of a convoluted tube. The female has an ovary, vagina, and uterus. The adult worm has an intestinal canal from end to end, which is divisible into oesophagus, stomach, and intestine.

Fig. 198. - Trichina spiralis in muscle. The elongated shape of the cysts is dvie to the fact that these were near the insertion of the muscle into its tendon. In the lowest, specimen the worm is dead and calcined.

The impregnated ova pass into the uterus where they develop into living embryoes of minute size. In six or seven days after the female has attained sexual maturity, that is, eight or nine days after the trichinous muscle has been eaten, the birth of living embryoes begins. The female gives birth to large numbers, and probably continues to do so for some weeks, thus producing as many as 1000 to 1300. The adults do not live longer than five to eight weeks altogether.

The minute embryoes now begin to penetrate the intestinal canal, and they swarm outwards to the voluntary muscles. The route by which they reach the muscles is not absolutely certain. By most it is thought that they pass outwards into the peritoneal cavity, and thence into the connective tissue around, by which they travel to the muscles. By others it is thought that they pass into the sub mucous connective tissue, thence into the connective tissue of the mesentery, and so onwards. It is probable that they find their way by both these routes, but it is inconceivable that, as some suppose, they get into the blood-vessels, as the vessels available are the portal radicles which would take them to the liver.

Swarming'outwards from the intestine they reach first the muscles of the trunk, where they are usually most abundant; they then pass to those of the neck and larynx; and, lastly, to those of the limbs. Arrived at the muscles they grow larger, and apparently wander about for a time. They penetrate inside the sarcolemma of the primitive fibre of the muscle, and destroy the sarcous substance. In about fourteen days they have attained their full size, and begin to settle down. As they pass along inside the sarcolemma they are arrested at the insertion of the fibre into the tendon, hence they are particularly numerous near tendons, and here also the cysts, subsequently formed, are often much elongated (as in the figure). The sarcolemma collapses as the sarcous substance is destroyed, and as the worm coils itself up spirally the sarcolemma forms for it an oval cyst. The worm itself also adds to the cyst a layer of its own. It is not uncommon to find two, or even more worms, in one cyst. In the muscles the worms assume a quiescent state and may remain so for years (as long as eighteen years has been proved), the cyst being impregnated with lime. They produce considerable destruction by piercing the sarcolemma, and disintegrating the sarcous substance, and there is often to be found a germination of the muscle nuclei around the worm. The death of the host does not cause the death of the trichina?. They will live in putrid flesh for weeks and remain capable of further development.

During the migration of the embryoes considerable irritation is produced. There is in the first week intestinal catarrh (diarrhoea), with fever, and the case may be mistaken for typhoid fever. Later, the muscles become stiff' and painful, and oedema of the skin, especially of the face, may develop. This oedema of the face, which occurs about the seventh day, is said to be of special diagnostic significance. The symptoms are usually at their height in the fourth or fifth week, and death occasionally ensues.

Besides in man, trichinae have been found in the muscles of the pig, cat, rat, mouse, marmot, polecat, fox, marten, badger, hedgehog, and racoon. By some it is believed that the rat forms the permanent source of infection, as, when one of these animals dies, it is eaten by its neighbours, and so the infection spreads. From their habits, it will be understood how swine sometimes partake of dead rats. The parasite will be communicated to man by eating imperfectly cooked swine's flesh. It is said that a temperature of 50°-55° C. (or 120°-130° F.), is enough to kill the em-bryoes, but it is quite conceivable that when large pieces of flesh are cooked rapidly, some parts may escape the thorough penetration of the heat.

The search for trichinae in the muscles of swine before the flesh is sold is compulsory in some countries. For the examination pieces of muscle (preferably from the diaphragm and larynx) are snipped off with scissors and spread out in water on a microscopic slide. Some liquor potassae may be added to make the preparation more transparent. It is then to be examined with low magnifying powers and afterwards with higher. Several specimens should be prepared from each animal.