Chapter 15: Nervous System Organization and Biological Clocks

Nervous System Organization and Biological Clocks introduces the concept of feeding specialization, showing how animals adapt their feeding structures and digestive physiology to their ecological niches. Examples include the expandable jaws of snakes, the baleen plates of whales, and the raptorial forelimbs of mantises. The digestive system is divided into four main functions: ingestion, digestion, absorption, and egestion. The chapter outlines the anatomical components of the vertebrate digestive tract, including the mouth, esophagus, stomach, intestines, pancreas, and liver. Mechanical and chemical digestion are discussed in tandem, with detailed explanations of enzymes like amylases, proteases, lipases, and nucleases, as well as the roles of bile salts and hydrochloric acid. The authors describe the importance of the midgut (especially the small intestine) in nutrient absorption, emphasizing the structural adaptations—like microvilli and increased surface area—that optimize this process. Comparative physiology is highlighted throughout, with case studies such as herbivorous fish with elongated intestines, hindgut and foregut fermenters like horses and ruminants, and carnivores with high-acid stomachs. Microbial symbionts are presented as essential partners in digestion for many animals, particularly in cellulose breakdown. The chapter explains the role of the microbiome in producing short-chain fatty acids and essential nutrients. Digestive regulation is covered in depth, including the hormonal control of appetite and secretion. Hormones like gastrin, secretin, cholecystokinin (CCK), and ghrelin help coordinate digestive timing, enzyme release, and satiety. The chapter concludes by discussing the energetic costs of digestion—known as specific dynamic action (SDA)—and how factors like temperature, meal size, and diet composition affect digestive efficiency. Together, these concepts build a complete picture of how digestion integrates with broader physiological systems.