The circulatory system of man and the vertebrate animals can be considered as made up of a small number of organs or subordinate systems, which are easy to recognize anatomically, and the functions of which are on the whole quite distinct.

For a large number of problems, there will be some animal of choice, or a few such animals, on which it can be most conveniently studied.

In the numerous observations made in my laboratory upon this object, we have only once seen a combination of vessels in which there might be a direct communication between a small artery and a vein, though the two observers could not come to a final conclusion on the point.

The affinity of blood or pure haemoglobins for oxygen is a complex phenomenon, depending upon a number of conditions, the most important of which are temperature and hydrogen ion concentration.

In many organisms, including man, the mechanical respiration and the circulation of the blood are 'regulated' so as to correspond to the demand of the moment.

In man, the mechanical breathing is essential to life, and it is one of the old tests for death to see whether these movements have ceased completely.