The epithelium in the buccal cavity contains cells that secrete mucus and fluid. When these cells form a collective unit with a common duct, they are known as an oral gland.
Oral Glands in Fishes and Tetrapods
Fishes: Oral glands are rare in fishes.
Tetrapods: Oral glands are more common due to the need to moisten food without water. Salivary glands are the most prevalent but are not homologous across species.
- Salamanders: Have mucous glands on the tongue and intermaxillary glands on the palate.
- Reptiles: Possess various oral glands such as infralabial glands along the lips, lingual glands on the tongue, sublingual glands below the tongue, and proximal and nasal glands associated with the snout, along with palatine glands on the roof of the mouth.
Function of Oral Glands
Oral glands serve multiple functions:
- Lubricating food during transport.
- Maintaining healthy membranes.
- Neutralizing toxins from prey.
- Initiating digestion.
Distribution of Pancreatic Tissue in Vertebrates
The pancreas in vertebrates functions as both an exocrine and endocrine gland. It may not always be organized into a discrete organ.
- Cyclostomes (Lampreys and Hagfishes):
- Exocrine pancreas is dispersed throughout the submucosa of the intestine and liver.
- Larval cyclostomes have ductless follicles of the endocrine pancreas near the anterior part of the intestine.
- Hagfishes (Adults): Endocrine follicles develop as discrete clumps near the bile duct opening into the intestine, with a rich vascular supply.
- Lampreys: The endocrine pancreas forms a distinct patch of tissue near the bile duct, separated from the dispersed exocrine pancreas along the intestine.
- Elasmobranchs (Sharks and Rays): The pancreas may be dispersed along blood vessels within the liver or form a discrete gland with both exocrine and endocrine components.
- Bony Fishes: Have distinct exocrine and endocrine pancreas with clearly delineated pancreatic islets.
- Tetrapods: Both exocrine and endocrine pancreas are always present, typically located near the duodenum.
Comparative Anatomy of the Liver
All vertebrates possess a liver. In Amphioxus, a cecum similar to the hepatic portal system exists but is not a direct antecedent. The structure of the vertebrate liver is composed of hepatocytes and blood sinuses.
Bile Production and Storage
The liver produces bile, which emulsifies fats. Most vertebrates store bile in the gall bladder for release during digestion, although it is absent in some species.
Pancreas
Embryonic Development:
The pancreas develops closely alongside the liver, arising from two unpaired diverticula:
- The dorsal pancreatic diverticulum buds directly from the gut.
- The ventral pancreatic diverticulum is a posterior bud of the hepatic diverticulum.
Ducts and Secretion:
- The ducts from the pancreas empty into the duodenal portion of the intestine.
- Pancreatic juice, an alkaline exocrine product, contains:
- Proteolytic enzyme trypsinogen.
- Amylases for carbohydrate digestion.
- Lipases for fat digestion.
Pancreatic Islets:
Embedded in the pancreas are small pancreatic islets (Islets of Langerhans) that produce hormones:
- Insulin: Lowers blood glucose levels.
- Glucagon: Raises blood glucose levels.
Additional Glands
- Duvernoy’s Gland: Found in nonvenomous snakes, releasing serous secretion near the posterior maxillary teeth.
- Venom Gland: In venomous snakes, secretes a mix of chemicals for both offense and digestion.
- Birds: Most birds lack oral glands, but some passerine birds have them for mucus secretion.
Primary Salivary Glands:
- Mandibular Glands: Also known as submaxillary glands.
- Sublingual Glands: Located below the tongue.
- Parotid Glands: Situated at the angle of the jaws, with ducts opening into the roof of the buccal cavity.
- Additional Salivary Glands: Present in some species like dogs and cats, such as the zygomatic gland under the zygomatic arch.
Liver
The liver is the second-largest organ in humans after the skin, involved in various roles:
- Producing red blood cells during fetal life.
- Detoxifying and removing toxic substances from the blood.
- Emulsifying fats using bile released into the intestine.
- Storing and metabolizing carbohydrates, proteins, and fats.
Vascularization:
- Highly vascularized with arterial blood supplied via the hepatic artery.
- Venous blood supplied via the hepatic portal vein from the intestines and spleen.
Embryonic Development:
- The liver develops from the ventral evagination of the gut floor.
- Mesenchyme induces endoderm to differentiate into hepatocytes.
- Hepatic sinusoids form from the vitelline vein.
