The circulation of the secondary lobule of the human lung.

els has proved to be a useful method for the study of the capillary circulation of the lung. Until recenth’, we felt that studying the capillary circulation in the component parts of the primary lobule** would reveal all the aspects of the terminal arterial circulation. It has been found, however, that the primary lobule does not disclose all the details of the terminal arterial vessels and their capillary networks. To understand the terminal and capillary circulation in its entirety, it is necessary to study the secondary lobulet as one unit. Only by doing so is it possible to demonstrate the importance of the different types of vessels which play their part in supplying the alveoli of the secondary lobule with their capillary networks. The purpose of this paper is to demonstrate that there are three sources of blood supply to the secondary lobule from which arterial alveolar capillary networks are formed. By the use of corrosion models it has also been possible to demonstrate how the arterial vessels by continuous subdivision form the capillary networks. Although there appear to be certain recognizable patterns, these cannot be strictly categorized, as variations occur between the various forms. Study of the alveolar capillary network has impressed us again, as in earlier studies4 with the richness of the anastomoses exist-