This book is a reprint of an English translation of Cajal's original work, with abundant notes and commentaries by the editor. This text describes Cajal's fundamental contributions to neuroscience, which continue to be important today. It accurately details Cajal's ideas and data, and provides readers with the opportunity to learn what Cajal thought about his research career and the significance of his observations. Excerpts from Tello's memorial lectures also provide a contemporary view of Cajal's work.