When, on a more advanced cultural level, the structure of the body came to be more closely observed, a strong impetus was given towards a progressive differentiation of the corporeal soul. Certain parts of the body, in particular, were singled out as vehicles of the soul. Those that are separable from the body, such, for example, as certain secretions and the products of growth, received a sort of intermediate position between the corporeal soul proper and the breath soul. Chief among these was the blood. Among some peoples, particularly the Bantus of South Africa, the saliva rivals the blood in importance, possibly because of the readily suggested association with the soul that departs in the vapour of the breath. The blood soul, however, is by far the most universal and most permanent of these ideas. In its after-effects it has survived even down to the present. For, when we speak of a 'blood relationship' uniting those persons who stand close to one another through ancestry, the word 'blood' doubtless represents a sort of reminiscence of the old idea of a blood soul. To the dispassionate eye of the physiologist, the blood is one of the most unstable elements of the body, so that, so far as the blood is concerned, the father and mother certainly transmit nothing of a permanent nature to their descendants. More stable parts of the organism are much more likely to be inherited. But, in spite of the fact that blood is one of the most transitory of structures, it continues to be regarded as the vehicle of the relationship existing between members of a family, and even between tribally related nations. More striking expressions of the idea of a blood soul are to be found on primitive levels. In concluding the so-called blood brotherhood, the exchange of blood, according to prevalent belief, mediates the establishment of an actual blood relationship. In accordance with a custom which probably sprang up independently in many different parts of the earth, each of the two parties to the compact, upon entering this brotherhood, took a drop of blood from a small, self-inflicted wound and transferred it to the corresponding wound of the other. Since the drop of exchanged blood represents the blood in general--not merely symbolically, as it were, but in real actuality--the two who have entered into the alliance have become nearest blood relatives, and thus brothers.