506 HUMAN BIOLOGY 



Another possible explanation is that the disease itself is 

 not inherited but only the Hability to contract it. That is to 

 say, an individual potentially sensitive by reason of inherit- 

 ance may escape the influence of the environmental factor 

 and never reveal his latent tendencies. 



It may well be remarked that in spite of a very great deal 

 of experimental work many of the factors in the state of 

 hypersensitiveness are not yet understood. For example, 

 some of the most striking and disastrous instances are those 

 of people sensitive to horse serum as determined by their 

 reactions to the injection of diphtheria antitoxin. Those in 

 whom the sensitiveness is most acute have usually also been 

 subject to attacks of asthma when the dust from horses 

 has been inhaled. But many people injected with antitoxin 

 become very sensitive to further injections of horse serum 

 without showing any tendency, so far recognized, to develop 

 asthma on contact with horses. It is apparent that the 

 inheritance is but one of the factors, even though an impor- 

 tant one, which must be considered when we try to understand 

 disease conditions. 



DISORDERS AND DEFECTS OF THE CENTRAL NERVOUS SYSTEM 



Popular interest in inheritance, ahke of normal and 

 abnormal quahties, naturally reaches its highest when the 

 nervous system is considered. From the medical point of 

 view we are here deahng with the diseases referable to a 

 single organ. Gross defects of development occur and are 

 likely to be lethal before their general effects on function 

 can become manifest. Finer defects in structure may well 

 be common but may escape recognition. The brain and 

 spinal cord are affected in the course of infectious diseases 

 which are to be considered as general infections, and also 

 are the seat of infections primary in or affecting, chiefly 

 themselves, e.g. pohomyeHtis, encephahtis lethargica, and 

 cerebrospinal meningitis. With reference to these what has 

 been said with regard to the inheritance of immunity or 

 susceptibility to infectious disease generally doubtless has 

 some appHcation in principle but we have no specific knowl- 

 edge of inherited influences in the particular cases. The 

 functional disorders of the nervous system are manifest in 



