Raymond Pearl 
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the factors which contribute to the differentiation of the " general hospital popu- 
lation " from the "general population" of the same locality. Of some of them, 
however, we may be fairly certain. In the first place, there are many classes 
(social) of the general population which will never be represented in any significant 
proportion in the general hospital population. Furthermoi-e, the nature of the 
injury or disease from which an individual is suffering in many cases determines 
whether the individual shall be in a hospital. On this point, Greenwood*, in an 
able discussion of the matter, says : " Evidently the population of a general hospital 
will chiefly consist of: (i) persons acutely ill, (ii) those suffering from surgical 
injuries or diseases, (iii) sufferers from medical affections requiring special treat- 
ment. Chronic maladies of old age, such as bronchitis, indeed, any highly chronic 
disease, will be under-represented in comparison with the general death-rate. 
Similarly, the number of cases of valvular heart disease and rarer disorders, such 
as Diabetes Mellitus or Insular Sclerosis and other nervous lesions, will be above 
the general average." 
In addition to the selection which occurs in the formation of the general 
hospital population there is a still farther weeding out when we come to deal with 
autopsy records, for the reason that not every individual dying in a hospital is 
subjected to a post-mortem examination. Whether there shall be an autopsy or 
not in a given case depends on several factors, one of which is the cause of death. 
The individuals whose last illnesses have given doubtful or rare clinical features 
are more likely to be subjected to a post-mortem examination, other things being 
equal, than the individuals in whose case the fatal illness has run a perfectly 
typical clearly cut course of some common disease. Consequently there is a 
tendency for the statistics to be unduly weighted with deaths from some of the 
rarer maladies. There can be no doubt that the statistical constants relating to 
any character of the body which is in any way affected by the disease causing 
death will be different, in material coming from hospital and pathological laboratory 
autopsy returns, from those which would be obtained could we get a perfectly 
random sample of the general population dying outside a hospital, and subjected 
to post-mortem examination regardless of the cause of death. 
These peculiarities attendant upon hospital autopsy records are of particular 
importance when the character studied is the weight of the brain, for the reason 
that this character is definitely affected both by the age of the individual, 
and, in certain cases, by the nature of the disease causing death. It is evident 
that there will be a more or less definite relation between the nature of the cause 
which brings an individual into a general hospital and the age of the individual. 
This combined relation of age and disease is a factor of importance in analysing 
brain-weight data. In what manner will be apparent from a moment's con- 
sideration. Suppose we take the first of Greenwood's classes of the general 
* Greenwood, M. : " A First Study of the Weight, Variability and Correlation of the Human 
Viscera, with special reference to the Healthy and Diseased Heart." Biometrika, iii. pp. 63 — 83, 
1904, p. 65. 
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