Human Anatomy - Kotsan I. Y. 2009

Splanchnology (The Doctrine of Internal Organs)
Large Glands of the Digestive System
A Brief Comparative Anatomical Overview of the Development of the Digestive System

The Digestive System is one of the most evolutionarily ancient systems. The primitive digestive pouch in Multicellular Organisms transforms into a tube (in worms) which, as organisms evolve, increases in length, develops a more complex wall Structure, and forms dilations, loops, and bends.

In the Cytology/cytology/16.html">Early stages of Phylogenetic and Ontogenetic development, the digestive and respiratory Organs constitute a single system closely linked with large digestive glands and the lower part of the Urogenital apparatus.

In primitive Chordates (such as amphioxus), the Digestive apparatus is shaped like a tube beginning with the Mouth and ending with the anal canal. They lack a respiratory apparatus; instead, the gas exchange function is performed by the anterior section of the intestinal tube (the pharyngeal wall) located near the mouth. In more advanced vertebrates, special respiratory organs—gills—develop on both sides of the intestinal tube near the mouth. At subsequent stages of evolutionary development, land-dwelling animals emerge, leading to The Development of air-breathing organs—Lungs—instead of gills. The validity of this proposition is supported by the fact that transitional animals still exist today, bridging aquatic and terrestrial life by possessing both gills and lungs, such as certain fish and amphibians (lungfish).

The structure of the digestive system in various animal species is related to their feeding habits, as well as how they obtain and process food. Cyclostomes lack jaws. Jaws first form in cartilaginous and sturgeon Fishes, already equipped with Teeth, which causes the mouth opening to shift to the ventral side of the HEAD. Lips are absent. The Tongue is poorly developed and lacks intrinsic Muscles. Intrinsic tongue muscles first appear in amphibians. The branchial apparatus associated with the Oral Cavity, which Functions as a respiratory organ in fish, transforms into other structures as animals transition to land during phylogenesis. In terrestrial vertebrates, respiratory organs develop whose epithelial lining originates from the primitive gut.

A partial Separation of the oral cavity into the oral cavity proper and the Nasal cavity occurs in reptiles. In these animals, the tongue is well-defined, and its size and shape correlate with their structural and behavioral adaptations. Labial and sublingual glands make their appearance.

In mammals, the oral cavity—into which the ducts of the Salivary Glands open—is completely separated from the nasal cavity, with The formation of the palate and the oral vestibule. The oral fissure is surrounded by relatively well-defined lips. The tongue and teeth are developed in accordance with the animal's feeding habits. The Esophagus in vertebrates, which serves to transport food to The Stomach, varies in length depending on the size and body STRUCTURE OF THE animal.

The shape and position of the stomach, along with the structure of its mucous and muscular membranes, are correlated with the animal's body shape and, notably, its feeding habits, the quantity and type of food consumed, and the development of other organs. In vertebrates with an elongated body, the longitudinal axis of the stomach extends along the body's axis; in short-bodied animals, the stomach is dilated and positioned obliquely or transversely in the Abdominal cavity. In birds, the stomach comprises glandular and muscular regions. In rodents and carnivorous mammals, the stomach is a simple dilation of the digestive tube. In herbivorous mammals, the stomach structure is complex, especially in ruminants, which possess a fairly large stomach divided into several distinct compartments. However, a high degree of gastric differentiation depends not on The complexity of its shape, but on the microscopic structure of its walls and the development of gastric glands.

In lower vertebrates, the intestine extends along the spine, whereas in higher-organized forms, it is convoluted into loops. In a range of vertebrates starting from higher fish, the cloaca separates into two distinct parts: the rectum and the urogenital sinus. Intestinal bends begin to form in amphibians, and their number increases in reptiles, accompanied by the division of the intestine into small and large intestinal regions. Only the terminal part of the intestine remains straight across all vertebrates. Birds have a long intestine with well-developed glands, which first appear in cartilaginous fishes. In herbivores, because plant matter differs significantly in chemical composition from animal Tissues, the intestine is considerably elongated. The Large Intestine achieves particularly prominent development; in some animals, such as the horse, additional caeca appear where Fermentation of undigested food residues takes place. In carnivores, the intestinal length is significantly shorter. Omnivores occupy an intermediate position regarding the structure of their digestive tract, and humans belong to this group.

The boundary between the small and large intestine is marked initially by a mucosal fold, which appears in certain amphibians, and subsequently by a valve. The large intestine begins with the cecum, which forms as a blind pouch in amphibians. This intestinal section is large in herbivorous mammals, but poorly developed or completely absent in carnivores. The vermiform Appendix is present only in certain mammals, such as rabbits, apes, and humans.

During the Formation of the small and large intestines, folds and glands appear within them, including large ones such as The Liver and Pancreas. The liver is present in all vertebrates. In some fish and amphibians, it is lobed and features appendages. Reptiles typically have a liver consisting of two lobes. A bilobed liver structure is characteristic of birds and mammals. The pancreas is small in fish, increases in size in amphibians, and is embraced by the loop of the developing duodenum in birds.