In human digestive physiology, succus entericus (intestinal juice rich in digestive enzymes) is secreted by which of the following intestinal structures?

Introduction:
The small intestine is the main site of digestion and absorption in the human body. Its inner lining secretes an intestinal juice known as succus entericus, which contains numerous enzymes that complete the digestion of carbohydrates, proteins and fats. This question asks you to identify the specific intestinal structures responsible for secreting succus entericus from among several anatomical features present in the intestinal wall.

Given Data / Assumptions:
- The secretion named is succus entericus, also called intestinal juice. - The options include Auerbach's plexus, Peyer's patches, Brunner's glands and crypts of Lieberkuhn. - We assume normal histology of the small intestine. - The focus is on the glands that actually produce and secrete digestive enzymes into the intestinal lumen.

Concept / Approach:
Succus entericus is secreted by the intestinal glands located in the mucosa of the small intestine. These glands are known as the crypts of Lieberkuhn. They produce a watery, enzyme rich secretion that bathes the villi and helps in final digestion at the brush border. Auerbach's plexus is a nerve plexus present between muscle layers and controls gut motility, not secretion. Peyer's patches are lymphoid aggregates involved in immune defence. Brunner's glands, found in the duodenum, secrete alkaline mucus to protect the mucosa from stomach acid, but they are not the main source of succus entericus. Therefore, the crypts of Lieberkuhn are the correct answer.

Step-by-Step Solution:
Step 1: Recall that succus entericus is the intestinal juice containing enzymes such as maltase, sucrase, peptidases and lipases. Step 2: Remember that these enzymes are secreted by the epithelial cells lining the intestinal glands in the mucosa. Step 3: Identify these glands by their histological name, crypts of Lieberkuhn, which are invaginations found between villi. Step 4: Examine the other options: Auerbach's plexus is neural, Peyer's patches are lymphoid and Brunner's glands secrete mucus, not the enzyme rich intestinal juice. Step 5: Conclude that the crypts of Lieberkuhn are responsible for secreting succus entericus.

Verification / Alternative check:
Textbooks of human physiology describe the small intestine mucosa as having villi and intestinal crypts. The crypts of Lieberkuhn are specifically mentioned as sources of intestinal juice and contain various cell types including enterocytes, goblet cells and Paneth cells. Auerbach's plexus (myenteric plexus) is highlighted for control of peristalsis, while Peyer's patches are shown as lymphoid nodules. Brunner's glands are noted as duodenal submucosal glands secreting alkaline mucus. These consistent descriptions verify that succus entericus is associated with the crypts of Lieberkuhn.

Why Other Options Are Wrong:
Auerbach's plexus: A network of autonomic nerve fibres between the circular and longitudinal muscle layers; it regulates intestinal motility rather than secretion of digestive juice. Peyer's patches: Lymphoid follicles in the ileum that play a role in immune surveillance of the intestinal lumen; they do not secrete digestive enzymes. Brunner's glands: Submucosal glands in the duodenum that secrete alkaline mucus to neutralise gastric acid; they are not the primary source of succus entericus.

Common Pitfalls:
Students often confuse the roles of different intestinal structures because they are all located close together in the intestinal wall. Brunner's glands and crypts of Lieberkuhn are both glandular, which can cause confusion about which one secretes intestinal juice. A useful memory tip is to associate Brunner's glands with protection (alkaline mucus) and crypts of Lieberkuhn with digestion (enzymes). Also, remember that any plexus refers to nerves, not glands, and patches usually refer to immune tissue rather than secretory cells.

Final Answer:
Succus entericus (intestinal juice) is secreted by the Crypts of Lieberkuhn in the small intestine.