Cells are composed of organelles and other cell compartments that work together during normal cell function. In multicellular organisms, cells join to form tissues. This section of the Atlas will deal with how tissues are organized to form organs in animals. Animals have a variety of organs, and each one develops particular functions. When comparing an animal with a cell, it can be noticed that functions are carried out by specialized compartments, but at different scales. In eukaryotic cells, these compartments are mostly organelles, whereas in animals, the compartments are organs and systems.
The following pages deal with the organs of vertebrates, mainly mammals (Figure 1). Mammals are probably the most studied vertebrates because of their similarities with humans and their impact on the knowledge of human organs and health. An organ is a functional structure of the body. As it usually happens in biology, it is sometimes difficult to delimit the precise role of an organ, and then we consider systems. Systems consist of groups of organs and structures that perform a function. For instance, the circulatory system. An organ, or a system, may take part in different functions or share functions with other organs.
Depending on the authors, there are different ways to study the animal organs and systems. Here, we will follow the syllabus of courses dealing with the microscopic study of animal organs, taught at some Spanish universities, with some changes.
