I thought that this excerpt from Alice Bailey's "Treatise on White Magic" gave a succinct description of the soul-body interface:
"It should be remembered that the purpose and will of the soul, the spiritual determination to be and to do, utilises the thread soul, the sutratma, the life current, as its means of expression in form. This life current differentiates into two currents or two threads when it reaches the body, and is "anchored", if I might so express it, in two locations in that body. This is symbolic of the differentiations of Atma, or Spirit, into its two reflections, soul and body. The soul, or consciousness aspect, that which makes a human being a rational, thinking entity, is "anchored" by one aspect of this thread soul to a "seat" in the brain, found in the region of the pineal gland. The other aspect of the life which animates every atom of the body and which constitutes the principle of coherence or of integration, finds its way to the heart and is focussed or "anchored" there. From these two points, the spiritual man seeks to control the mechanism. Thus functioning on the physical plane becomes possible, and objective existence becomes a temporary mode of expression. The soul, seated in the brain, makes man an intelligent rational entity, self-conscious and self-directing; he is aware in varying degree of the world in which he lives, according to the point in evolution and the consequent development of the mechanism. That mechanism is triple in expression. There are first of all the nadis and the seven centres of force; then the nervous system in its three divisions: cerebro-spinal, sympathetic, and peripheral; and then there is the endocrine system, which might be regarded as the densest aspect or externalisation of the other two.
The soul, seated in the heart, is the life principle, the principle of self-determination, the central nucleus of positive energy by means of which all the atoms of the body are held in their right place and subordinated to the "will-to-be" of the soul. This principle of life utilises the blood stream as its mode of expression and as its controlling agency, and through the close relation of the endocrine system to the blood stream, we have the two aspects of soul activity brought together in order to make man a living, conscious, functioning entity, governed by the soul, and expressing the purpose of the soul in all the activities of daily living."