Which is NOT an accessory organ to the digestive tract of a horse?

The kidney is classified as an organ involved in the excretory system rather than the digestive system, which is why it is identified as NOT an accessory organ to the digestive tract in a horse. Accessory organs in the context of digestion are those that aid in the digestive process but are not part of the main digestive tract. This includes the liver, which produces bile for fat emulsification; the pancreas, which secretes digestive enzymes and hormones; and the spleen, which has roles in the immune response and the management of blood, but does not directly participate in digestion.

In contrast, while the kidneys play a critical role in filtering blood and managing waste products and electrolytes, their function does not directly relate to the breakdown or absorption of food, which is the primary purpose of the digestive tract and its accessory organs. Understanding these distinctions is crucial in horse anatomy and physiology, particularly when studying digestive processes.