Mouth digestion

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As Maul digestive be in animal physiology all the operations referred to in food intake in the oral cavity take place. This includes the ingestion, chopping and salivation of food, which vary in detail depending on the food spectrum. A significant enzymatic breakdown of the food does not take place in animals, only starch is broken down in some animal species by a starch-breaking enzyme ( amylase ).

Consumption of solid and liquid food

Feed intake is a prerequisite for the digestive processes to proceed. There are different images of the process of feed intake between the individual animal species, which are best made accessible through personal observation.

In cattle , the lips are only slightly mobile. Fodder parts are grasped by the tongue and guided between the incisors of the lower jaw and the dental plate of the intermaxillary bone so that they can be held there. With jerky movements of the head, grass is then torn from the ground or roughage removed from the larger pile.

In small ruminants such as sheep or goats , on the other hand, the split lips are much more mobile, so that they are of much greater importance for grasping the food and feeding it to the incisors. As a result, small ruminants are able to select significantly more when it comes to feeding than cattle . The pointed shape of the mouth also enables them to selectively pick up soft plant parts that can be found between heavily lignified or even thorny material. Horses grab parts of food mainly with their very flexible and strong lips. Pasture grass or long forage is pushed between the incisors, with which it is bitten off, with a slight jerk of the head having a supportive effect.

Rummaging through the feed with the proboscis is typical for pigs , whereby the lips and proboscis are of crucial importance for grasping meal-like or pulpy feed. If grass or roughage is used, for example in sow husbandry, the pig uses its incisors to bite off or hold. Dogs and cats grasp and hold solid food with their hook and incisors, as is the case with marsupials . In order to be able to bite off or tear off absorbable parts, the shoulder extremities are often used. Soft or mushy food is taken in with the tongue. Newborns, for whom breast milk is usually the only food, feed by suckling on the mother's udder . The teat is brought between the roof of the mouth cavity and the surface of the tongue. By rhythmically opening the jaws and moving the tongue backwards, a negative pressure is created in the oral cavity , which leads to the passage of milk from the cistern of the udder into the oral cavity.

In the case of herbivores and omnivores , fluid is absorbed even with advancing age by sucking after dipping the lips into the fluid. If, in addition to liquid, air is sucked into the oral cavity to a significant extent, as is often the case with pigs, the typical "slurping" occurs. Carnivores only suckle on the mother's udder, in later life they only use the tongue to absorb fluids. Dogs scoop with the spoon-shaped tip of the tongue. In cats, fluid adheres between the particularly large number of papillae on the tongue. Cats ingest fluid through repeated immersion and withdrawal into the oral cavity.

Formation of swallowable bites

After being taken up in the oral cavity, solid food has to be mechanically changed to a greater or lesser extent before it can be swallowed. Movement of the lower jaw against the fixed upper jaw causes the chucked parts to be comminuted between the molars . In carnivores, this movement of the lower jaw takes place almost exclusively in a vertical plane; in herbivores, on the other hand, also in a horizontal plane, so that the chewing movement is referred to as the "grasping type" in the former and the "grinding type" in the latter. The tongue pushes the pieces of food that have fallen from the chewing surface of the teeth again and again between the rows of molars. In carnivores, the upper and lower jaws have approximately the same width, while in herbivores the lower jaw has a significantly smaller width than the upper jaw. As a result, herbivores can only chew on one side at a time, with the preferred side changing continuously. While the dog essentially restricts chewing to breaking up bones and swallowing larger bites quickly, the cat chews very thoroughly. The horse chews particularly thoroughly, as it takes three to four times as long as cattle to consume comparable amounts of roughage. In addition to being crushed by chewing ("mastication"), salivation is an essential part of the digestive system. The saliva is a mixture of secretions from various wall glands and the three appendage glands arranged in pairs. On each page are:

Two cell types can be distinguished in the glands . Some secrete liquid that contains not only electrolytes but also protein in the cell water and is called serous . The others are able to add mucin ( mucins ) to a basically similar product , which is why this is referred to as a mucous secretion. This mucilage creates the stringy consistency of the mouth saliva. Only cells of the first type are found in the parotid glands, so they secrete seromucosal saliva .

Excitations triggered by mechanical or chemical stimuli at the nerve endings of the sensitive nerves in the oral mucosa are directed to the center of saliva secretion in the brain stem. There, the switch to sympathetic or parasympathetic fibers takes place, via the composition and quantity of the secretion of the salivary glands are controlled.

In addition to this unconditional reflex control of the secretion of mouth saliva ( salivation ), there are also conditional reflex controls , which are particularly well developed in dogs. When certain phenomena are perceived, which experience has shown to coincide with the administration of food, stimuli that have reached the cerebrum via olfactory, facial or auditory nerves are passed on to the center of saliva secretion, so that salivation begins.

Except in ruminants, the secretion of large amounts of mouth saliva only takes place during feed intake. In ruminants, a small amount of saliva flows continuously, but this increases when they eat food and of course when they start chewing.

The saliva of pets is alkaline with pH values in the range between 7.2 and 8.4. The organic compounds in saliva include proteins and glycoproteins as well as urea . In comparison to blood plasma , the saliva in ruminants is approximately isotonic , but in other domestic animals it is hypotonic .

With demand utility services under the cations the sodium quantitatively by far in the first place, in addition to which low concentrations in relatively also potassium , magnesium , calcium found. At the same time, the saliva contains hydrogen carbonate , phosphate and chloride as important anions . If, as a result of inadequate sodium supply, ruminants become depleted of this element, the concentration of sodium is, however, greatly reduced with a compensatory increase in the potassium concentration.

For this reason, a Na / K ratio in the saliva of cattle that deviates from the norm can be regarded as an indication of a sodium deficiency in these animals. Digestive enzymes are secreted in the mouth saliva of domestic animals to a negligible extent , although a certain activity of amylase can be detected in the saliva of the pig.

The knowledge of the daily amount of saliva secreted is assured to different degrees for different animals. However, it can be assumed that ruminants, which have the largest amounts of saliva in comparison, are in the range of around 8 to 14 liters per kilo consumed. This means that a cow that eats 15 kilos of dry matter a day secretes between 100 and 200 liters of mouth saliva.

Swallowing the bite

Chewing not only breaks up the portion of food, it is also intensively mixed with the saliva secreted in the oral cavity. In this way, the feed is partially softened as a result of swelling processes; on the other hand, the mucins in the saliva make it more slippery and thus easier to swallow. Only through this change is swallowing (deglutition) possible with many feeds . During the act of swallowing, the prepared bite or the liquid is initially arbitrarily pressed from the back of the tongue against the hard palate . Then a curvature of the base of the tongue transports the material to be swallowed into the pharynx .

In the area of ​​the isthmus faucium there are sensitive nerve endings, so-called swallowing points, which cause the act of swallowing to proceed in a reflex, i.e. involuntary manner. So that the bite gets into the esophagus and not the airway, the soft palate is placed horizontally so that it rests against the posterior wall of the pharynx. The larynx is raised and closed by the lid of the larynx. At the same time, the initial part of the esophagus opens like a funnel. In this phase the food pathway is open, but the airway through which it crosses is closed. so that there is a brief interruption of breathing when swallowing.

The act of swallowing takes place in the transport of the bite or swallowing through the esophagus into the stomach. The necessary peristaltic movements of the striated muscles of the esophagus are triggered either reflexively with the act of swallowing or after local irritation of the esophagus by the bite. While the muscles on the stomach side of the bite slacken, there is a ring-shaped constriction on the mouth side, which continues in a wave-like manner towards the stomach and thus allows the bite, also known as a " bolus ", to slide towards the stomach. The transition from the esophagus to the stomach is formed by a sphincter muscle , which is most strongly developed in horses. The swallowing act is regulated by a central nervous swallowing center, which receives excitation via sensory fibers, which leads to motor and autonomic fibers, which then cause the muscles involved to contract in the necessary sequence.

literature

  • Argenzio, RA - The large bowel, a supplementary rumen?
  • Harrison, FA - Digestion and absorption of lipids in non-ruminant animals
  • Giesecke, D. - Biology and Biochemistry of Microbial Digestion