If we define the Constitution as the inherent structure and powers of the organism, then we shall recognize that the constitution of the individual is made up of an immense number of particulars of structure and function, each of which has been transmitted from a parent or ancestor. These particulars include susceptibilities to disease, so that we speak of constitutional susceptibility. As we have seen, each of these constitutional peculiarities is inherent in the fertilized ovum when it enters on its career of development.

It is proper here to notice that Constitutional Disease is to be carefully distinguished from constitutional susceptibility. The former implies some serious disorder which has involved the bodily structures and powers as a whole, such as syphilis, gout, and, in some cases, cancer. From this circumstance it is often inferred that there is a contradistinction between constitutional and local, whereas really the constitution is made up of local peculiarities, and constitutional susceptibility is mostly local in its nature.

The term Diathesis is sometimes used in a sense nearly equivalent to constitutional disease, but with an implication that there is a constitutional predisposition before the actual occurrence of the disease. Thus a gouty diathesis is supposed to precede as well as to coexist with the actual occurrence of gout. The term, however, is not now much in use, and is not capable of very strict definition.

Besides the constitution of the body as a whole we are able to speak of the Constitution of the various organs and tissues of the body, and this is manifested in variations of susceptibility to disease. This may be illustrated by contrasting the susceptibility of the lungs and liver respectively to certain forms of disease. Cancer is a disease characterized by an excessive multiplication of epithelial cells, and it frequently happens that the original tumour sends out offsets, presumably in the form of cells, into structures at a distance, these being in some cases carried by the blood. When implanted in the liver these offsets usually grow luxuriantly, so that there may ultimately be much more cancerous than liver tissue. On the other hand, the same offsets planted in the lungs will grow very little.

There is often abundant microscopic evidence of their implantation, but no proper tumours. The lungs and liver are contrasted in an opposite sense in the case of general tuberculosis. In this disease the specific microbe is present in the blood, and is planted throughout the organs and tissues. In the liver it is caught more frequently than probably in any other organ, but it produces only microscopic lesions. These lesions are in immense numbers, but they are rarely visible to the naked eye, and are probably of little consequence to the organ; the microbe is in an uncongenial situation. In the lungs, on the other hand, each implantation leads to a marked lesion of some size, and showing evidences of much disturbance of the tissue. The syphilitic poison presents characteristics different from that of tuberculosis* and similar to that of cancer. The liver is frequently affected, the lungs very rarely.

These illustrations might be multiplied in regard to other organs and tissues. That such differences in susceptibility are the result of inheritance is shown by the fact that they are not identical in the different species of animals. Thus, in the guinea-pig and in the ox the microbe of tubercle when implanted in the liver produces obvious and considerable lesions similar to those in the lungs.