series, the incubation period becomes gradually shortened until it finally reaches a fixed point of from six to seven days ("virus fixe"). As a rule, this is the strongest and most stable virus that can be obtained.

If the medulla from a rabbit that has died after inoculation with this fixed virus be preserved in a dry atmosphere, its toxicity gradually becomes lessened as the drying proceeds, and totally disappears in about two weeks if the drying has been at a temperature of from 23° to 25° C. With the medulla so treated the period of incubation, as determined by the inoculation of rabbits, becomes longer as the virus becomes attenuated in the degree of its potency. In Pasteur's protective inoculation, the treatment is begun with the subcutaneous injection of emulsions of spinal cords or medullæ that have been thus dried until almost robbed of their toxic properties; this is followed by similar injections of emulsions from more and more toxic tissues, until finally the animal is prepared for an injection with the strongest-i. e., "fixed virus;" when this stage is reached, protection is assumed to be completed.

For the successful employment of this method it is essential that it be begun at the earliest possible moment after the reception of the virus-i. e., after the bite of the rabid animal. The greater the delay the less likely is the treatment to be effectual.

The influence of this mode of treatment upon the mortality from hydrophobia among human beings bitten by rabid animals is so striking as to efface all question as to its efficacy. As stated above, a fair average mortality for all cases of bites from mad animals is 15 per cent., while the mortality among those bitten upon the head, face, neck, and hands, ranges from 60 to 80 per cent. The following table, compiled by Pottevin, illustrates the striking reductions in these deathrates that have been accomplished through this mode of treatment in the Institute Pasteur at Paris.

Table showing the results of the Pasteurian preventive inoculation against hydrophobia in human beings, for the twelve years

It is manifestly inadvisable to attempt in a book of this character the detailed treatment of so large a subject. The following must, therefore, be regarded as a mere sketch of the more frequent and important conditions observed in man as a result of the invasion of the body by the commoner animal parasites.

As has been already intimated in the first section, the disturbance of physiological function and the morbid conditions that arise from the presence of entozoa in the body are in the main due to their character, mode of development, their local irritation, to the demands that they make upon the blood and tissues for nutrition, and to their obstructive action when located in important viscera and organs of special sense. With the exception of the blood-sucking ankylostoma duodenale,

1 See Les Vaccinations Antirabiques à l'Institut Pasteur en 1898, par Henri Pottevin; Annales de l'Institut Pasteur, 1899, tome xiii., p. 518.

those that occupy the alimentary tract cause only disturbances of the digestive processes and reflex nervous phenomena, and are not accompanied by definite or serious structural lesions. Those located within the tissues cause irritation which may in time be followed by more or less of inflammatory reaction. Certain of these, by developing to an unusual size (echinococcus cyst), may seriously obstruct the functional activity of the organ in which they are located; while those occupying a position in such important vital parts as the central nervous system, the organs of special sense, and the circulatory ap paratus, may result in the gravest disturbances, and even in death.

Diseases of this class, and especially those in which the parasite resides within the alimentary tract or internal viscera, are most frequently contracted through the swallowing of either the mature parasite or its eggs, or its larval forms.

As a rule, the commonest worms of man enter the body as larvæ with the flesh of animals in which the first stages of development of the worm have taken place; or in some instances they may be derived directly from domestic animals whose flesh is not used as food, but with which the individual may have been in intimate association; or in other cases they may be traced to the eggs of parasitic worms that are contained in polluted drinking water.

The Nematoda.-These worms are so called because of their filiform or thread-like shape. The commonest spe

cies are:

Ascaris Lumbricoides (Round Worm).-It is in general similar in appearance to the ordinary earth-worm. The females measure about 30 cm. (12 inches), and the males about 15 cm. (6 inches) in length. It locates in the small intestine of man, and, as development proceeds, a large number of eggs are passed. The eggs are brownish in color, as seen in the stools, and are of a barrel shape. The eggs are carried by water, and it is through the drinking of water polluted with them that the individual often becomes infected. The worm is also seen in swine, and for this reason water polluted with hog excrement should be viewed with suspicion.

Oxyuris Vermicularis (Thread Worm).-This worm invades the cecum and upper colon. The female, which is about 12 mm. in length, deposits eggs in very large numbers. Within these eggs spermatozoa-like embryos may often be detected. When swallowed, the envelope of the embryo is digested by the gastric juice and the larvæ liberated. They pass through the early developmental stage in the upper intestine and reach maturity in from twenty-five to thirty days. Raw vegetables or insufficiently cooked vegetables are said to be the most frequent source of infection. The larvæ are destroyed by long immersion in water.

Trichocephalus dispar (whip-worm) is the commonest intestinal parasite of man in tropical latitudes. It retains its vitality in water and in moist soil. The embryos in the egg are very tenacious of life. When the egg is swallowed, the larva is liberated in the alimentary canal, attaches itself to the wall of the intestine by its lash-like extremity, and proceeds to develop slowly, reaching maturity after about ten or twelve months.

Ankylostoma duodenale is a minute worm, which by its presence in the upper portion of the small intestine causes the disease called ankylostomiasis, or Egyptian chlorosis. The parasite becomes attached, often in enormous numbers, to the villi of the duodenal mucous membrane. Through the continuous draught that it makes upon the circulating blood there results the condition of anemia by which the disease is characterized clinically, and from which it takes its popular name. The parasite is apparently widely distributed, as the disease has been observed in Egypt, India, Italy, Africa, Peru, and Australia. An analogous disease, known as "miners' cachexia," "miners' chlorosis," "tunnel disease," has been observed among Italian workmen in the St. Gothard tunnel. It has also been known among Italian brickmakers and in those who have worked upon the irrigation fields. The disease is said to have been known for some years among the coal miners of Belgium and the brickmakers of Cologne as "brick-makers' anemia." According to Circular No. 5 of the Superior Board of Health of Puerto

Rico (1900), it is one of the most prevalent and fatal diseases of the island. One thousand death's from it were reported by 57 municipalities during October, 1899, and 692 by 36 cities in November of the same year. From the same circular it is interesting and important to note that the disease is not uniformly disseminated throughout the island. While conspicuous in some places, there are many others from which it is apparently absent. The development of this parasite seems to be favored by wet, marshy soil. In appearance the parasite is whitish or brownish in color, and in shape it is cylindrical, with more or less pointed ends. It measures from 6 to 18 mm. long and about 1 mm. in thickness. The male has a bell-like expansion at its posterior extremity. It attaches itself, as said, to the mucous membrane of the duodenum and upper jejunum, by means of a set of tooth-like hooks with which the mouth is provided. The eggs of this parasite are ejected with the feces, and probably undergo further development when favorable conditions, such as soil-moisture, high temperature, and free access to air, exist.

Infection probably occurs through the drinking of water from marshy soils on which the eggs are deposited with the feces of individuals in whom the entozoon is present, or in some one or another of the manifold ways that are open to men working in and upon such infected areas. It is almost exclusively seen in persons whose occupations bring them in contact with earth, as the various synonyms of the disease imply.

It is obvious that the precautions to be taken to prevent infection and dissemination are to drink only boiled or filtered water, pay particular attention to personal cleanliness, especially of the hands, and to disinfect carefully the stools from all individuals in whom the parasite is present.

Rhabdonema Intestinale. — Frequently associated with ankylostoma duodenale is the nematode rhabdonema intestinale, the parasite that is concerned in the causation of the so-called "diarrhea of Cochin China."

The lodgement of the female worm, which is about 2 mm,