Deiters drawings show two types of delicate processes attached to the neuronal cell body (a particularly beautiful example adorns the cover of this book). The shorter processes, which he called protoplasmic extensions, are today known as dendrites, and they typically branch like a tree. Dendrites are specialized appendages that receive signals emitted by other neurons. Deiters called the longer, thicker processes axis cylinders, now known as axons. Usually each neuron has a single axon. Axons conduct electrical pulses from one neuron to another. They often gather together in the thousands or millions to form ber bundles. Some bundles remain within the brain, while others become nerves, leaving the brain to connect with muscles and endocrine glands. Axons can be as short as a few micrometers or, in the case of axons that cause the toes to wriggle, about one meter long in an adult person of average height; the corresponding axons of a girae are much longer.

Deiters was not a psychiatrist, but a medical doctor with a small private practice. Most of his time was spent in the anatomy laboratory. Bernhard Gudden, by contrast was heavily involved in psychiatry, yet managed 121

10 A Very Complex Thing to conduct a substantial amount of neuroanatomical research in his spare time, as the saying goes. Guddens serious engagement with neuroanatomical research was directly inspired by Griesingers call for a science-based psychiatry, so it is reasonable to ask, what was the nature of this research, and what were its results? Anatomists had only a basic knowledge of the brain when Gudden began his research. They knew that it contained neurons, bers and blood vessels. The neurons had various shapes. Some were roundish, some star-like, and still others pyramidal. Regardless of their types, the neurons tended to mass together in countless numbers, but never so tightly that individuals could not be discerned when using a high powered lens. The bers were long and very thin and looked nothing like neurons. Individual bers could be seen scattered around neurons, but they were most impressive when massed together in bundles, also known as tracts.

Finding cells in the brain did not surprise Gudden and colleagues, because Rudolf Virchow, student of Johannes Muller, had shown that cells are present everywhere in all plants and animals. But, their overwhelming numbers in the brain puzzled the neuroanatomists and raised many questions. Why are they unevenly distributed throughout the brain? Why do they appear in dierent sizes and dierent shapes? And, most obviously, what exactly do brain cells do, and how do they do it?