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passing a sharply limited beam of X-rays, beta rays or y rays immediately underneath the drop, catching upon the drop the positive ion, formed by the ionization of a neutral molecule by the rays under investigation, and finally measuring the charge communicated to the drop by the advent of the ion upon it through observing the speed imparted to the drop by its new increment in charge.

Just before the war, Millikan attacked the more difficult and the more interesting problem of catching by the same general method, the ions formed by the passage of an alpha particle through an atom, expecting in this case to find that this relatively huge and powerful ionizing agent would often detach more than one negative electron from a single atom. When he was called to other duties by the war, the experimental work already begun was continued and completed by Gottschalk and Kelly. The results are as follows:

1. Alpha rays have been shot through atoms of the most diverse sorts (H, C, O, N, Cl, I, Hg) and of atomic weights from 1 to 200, without bringing to light in any case evidence of the formation of multiply-valent ions.

2. Twenty-nine hundred ions formed by the passage of a rays through neutral molecules have been caught on oil drops at the instant of ionization and the charges carried by each of these ions individually measured. Of these 2900 captures, 5 might possibly have corresponded to double charges, though even these were in all probability due to the nearly simultaneous advent upon the drop of two singly charged ions.

3. In no single case has an a particle been observed to form an ion carrying three or more charges, even though mercury, from which octivalent ions had been expected, was one of the substances tested.

4. Alpha ray ionization consists, then at least 99 times out of a 100, in the case of all the gases and vapors studied, in the detachment of a single negative electron from a neutral molecule.

1 A detailed report of these experiments will shortly be published in the Physical Review. 'Millikan, R. A., and Fletcher, H., London Phil. Mag., (Ser. 6) 21, 1911, (753).

ON THE EMBRYOLOGICAL BASIS OF HUMAN MORTALITY1

BY RAYMOND PEARL

DEPARTMENT OF BIOMETRY AND VITAL STATISTICS, JOHNS HOPKINS UNIVERSITY

Read before the Academy, November 11, 1919

1. In order to get a clearer idea of the underlying biological factors in human mortality I have rearranged the 'causes of death' listed in the International Classification of the Causes of Death, which is the code used generally by vital statisticians, into a new classification on a biological basis. It is not possible with our present statistical material to make a completely and precisely logical classification, but I have endeavored to come as close to it as is possible. The underlying idea of this new classification is, as the first operation, to group all causes of death under the heads of the several organ systems of the body, the functional breakdown of which is the immediate or predominant cause of the cessation of life. All except a few of the statistically recognized causes of death in the International Classification can be assigned places in such a biologically grouped list. It has a sound logical foundation in the fact that, biologically considered, death results because some organ system, or group of organ systems, fails to continue its functions. Practically, the plan involves the reassignment of all of the several causes of death now grouped by vital statisticians under heading 'I. General diseases.' It also involves the re-distributing of causes of death now listed under the puerperal state, malformations, early infancy, and certain of those under external causes.

The headings finally decided upon for the new classification are as follows:

I. Circulatory system, blood, and blood-forming organs

II. Respiratory system

III. Primary and secondary sex organs

IV. Kidneys and related excretory organs

V. Skeletal and muscular systems

VI. Alimentary tract and associated organs concerned in metabolism VII. Nervous system and sense organs

VIII. Skin

IX. Endocrinal system

X. All other causes of death

It should be emphasized that the underlying idea of this rearrangement of the causes of death is to put all those lethal entities together which bring about death because of the functional organic breakdown of the same general organ system. The cause of this functional breakdown may be anything whatever in the range of pathology. It may be due to bacterial infection; it may be due to trophic disturbances; it may be due to mechanical disturbances which prevent the continuation of normal function; or to any other cause whatsoever. In other words, the basis of the present classification is not that of pathological causation, but it is rather that of organological breakdown. We are now looking at the

TABLE 1

SHOWING THE RELATIVE IMPORTANCE OF DIFFERENT ORGAN SYSTEMS IN HUMAN

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question of death from the standpoint of the pure biologist, who concerns himself not with what causes a cessation of function, but rather with what part of the organism ceases of function, and therefore causes death.

2. In table 1 the death rates per 100,000 are arranged in descending order of importance (for the United States Registration Area 1906-10) by organ systems. Four sets of data are used: (a) the United States Registration Area for the five years, 1906–10 inclusive; (b) the same, 1901-05; (c) England and Wales, 1914; and (d) Sao Paulo, Brazil, 1917.

3. The data show that in the United States, during the decade covered, more deaths resulted from the breakdown of the respiratory system than from the failure of any other organ system of the body. The same thing

is true of England and Wales. In Sao Paulo the alimentary tract takes first position, with the respiratory system a rather close second. The tremendous death rate in Sao Paulo chargeable to the alimentary tract is chiefly due to the relatively enormous number of deaths of infants under two from diarrhea and enteritis. Nothing approaching such a rate for this category as Sao Paulo shows is known in this country or England.

In all three localities studied the respiratory system and the alimentary tract together account for rather more than half of all the deaths biologically classifiable. These are the two organ systems which, while physically internal, come in contact directly at their surfaces with environmental entities (water, food, and air) with all their bacterial contamination. The only other organ system directly exposed to the environment is the skin. The alimentary canal and the lungs are, of course, in effect invaginated surfaces of the body. The mucous membranes which line them are far less resistant to environmental stresses, both physical and chemical, than is the skin with its protecting layers of stratified epithelium.

The organs concerned with the blood and its circulation stand third in importance in the mortality list. Biologically the blood, through its immunological mechanism constitutes the second line of defense which the body has against noxious invaders. The first line is the resistance of the outer cells of the skin and the lining epithelium of alimentary tract, lungs, and sexual and excretory organs. When invading organisms pass or break down these first two lines of defense the battle is then with the home guard, the cells of the organ systems which, like the industrial workers of a commonwealth, keep the body going as a whole functioning mechanism. Naturally it would be expected that the casualties would be far heavier in the first two defense lines (respiratory and alimentary systems, and blood and circulation) than in the home guard. Death rates when biologically classified bear out this expectation.

It is at first thought somewhat surprising that the breakdown of the nervous system is responsible for more deaths than that of the excretory system. When one bears in mind, however, the relative complexity of the two pieces of machinery, it is perceived that the relative position of the two in responsibility for mortality is what might be reasonably be expected.

In the United States the kidneys and related excretory organs are responsible for more deaths than the sex organs. This relation is reversed in England and Wales and in Sao Paulo. The difference is mainly

due, in the case of England, to two factors, premature birth and cancer. In Sao Paulo it is due to premature birth and syphilis.

In a broad sense the efforts of public health and hygiene have been directed against the affections comprised in the first two items in the table, respiratory system and alimentary tract. The figures in the first two columns for the two five year periods in the United States indicate roughly the rate of progress such measures are making, looking at the matter from a broad biological standpoint. In reference to the respi

TABLE 2

SHOWING THE RELATIVE INFLUENCE OF THE PRIMARY GERM LAYERS IN HUMAN

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SHOWING THE RELATIVE INFLUENCE OF THE PRIMARY GERM LAYERS IN HUMAN

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ratory system there was a decline of 14% in the death rate between the two periods. This is substantial. It is practically all accounted for in phthisis, lobar pneumonia, and bronchitis. For the alimentary tract the case is not so good-indeed, far worse. Between the two periods the death rate from this cause group fell only 1.8%. 4. The next step in the investigation was to arrange all the organologically classifiable death rates under the primary germ layers (ectoderm, mesoderm, and endoderm) from which the organs concerned developed embryologically. The results are set forth in tables 2 and 3, and in

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