Note about the illustrations. Many of the following are digital images or drawn from either (1) whole mounts of subgross preparations stained with hematoxylin at low pH (so that only nuclei are colored), or (2) from routine histology slides stained with hematoxylin and eosin. The drawings permit representation of the full three dimensional field that is often impossible with the limited depth of focus with a camera. All drawn illustrations closely represent the subject as observed in the morphological preparations. The magnifications are shown as units of length marked directly on the drawings. The terms "acinus" and "alveolus" are synonymous. The medium for the drawings is pen, black ink and gray tone watercolor wash. The images were taken with a Kontronic camera and digitized using Adobe Photoshop.