There are two sources of digestive enzymes: The glandular cells of the lining of the stomach and intestine, which directly pour their secretion into the lumen of the gut or the alimentary canal. Special glands such as the salivary glands, the liver and the pancreas which pour their secretions into the gut through their ducts. There are three pairs of Salivary glands in our mouth cavity.

Salivary Glands:

Saliva is secreted by salivary glands. Saliva is watery fluid with pH in the range of 6.5 to 7.5. It consists of 98-99% water, 0.2 % salts and remaining digestive enzymes. Salivary glands secrete about 1000 to 1200 ml saliva per day.

• Parotid glands: They are located in front of and below each ear. They produce watery saliva rich in amylase (starch digesting enzyme). The parotid ducts (Stensen’s ducts) open into vestibule opposite the upper second molar teeth.
• Submaxillary glands: They are at the angle of the lower jaw. They produce water and mucus. The submaxillary ducts (Wharton’s ducts) open under the tongue.
• Sublingual glands: They are below the tongue. They produce water and mucus. The sublingual ducts (Duct of Rivinus) open under the tongue.

These glands continuously pour saliva into the mouth cavity.  Saliva drains into the mouth through small tubes called ducts. These glands secrete about 1000 to 1200 ml per day.

Functions of Saliva:

• It cleans the mouth cavity and tends to destroy germs with its lysozymes that cause teeth decay. Lysosomes act as antibacterial agents and prevent infection.
• The mucus in saliva moistens and lubricates food which helps in masticating, forming a bolus, and swallowing.
• It acts as a solvent, dissolving some food particles to stimulate taste buds of the tongue.
• Saliva helps in the digestion of food as it contains an enzyme salivary amylase which digests starch converting it into sucrose. That is why starch, when chewed, leaves a sweet taste in the mouth.
• Salivary amylase (ptyalin) hydrolyzes starch and glycogen into maltose, isomaltose, and small dextrins.
• Bicarbonates, phosphates, and mucin neutralize acids in the food and hence act as a buffer.
• Thiocyanate ions act as antimicrobials and prevent infection.
• Enzyme polysaccharidase destroys bacteria.

Problems/diseases Related with Salivary Glands:

• When there is a problem with the salivary glands or ducts, it results in symptoms such as salivary gland swelling, dry mouth (xerostomia), pain fever, foul-tasting into the mouth.
• Salivary stones, or sialoliths:  Salivary stones are formed due to the buildup of crystalized buildups. They block the flow of saliva into the mouth. When saliva is not released through the ducts, it backs up into the gland. It causes pain and swelling. To avoid the infection the gland should be cleaned.
• Salivary gland infection or sialadenitis. When the salivary glands get blocked, there is a chance of infection of these glands particularly the parotid gland. The infection creates a painful lump in the gland, and foul-tasting pus drains into the mouth. This infection may cause severe pain, high fevers, and pus in the mouth. It is common in older adults.
• Infections. Viral infections such as mumps and flu can cause swelling of the salivary glands. Swelling happens in parotid glands on both sides of the face. Mumps usually begins approximately 48 hours after the start of other symptoms such as fever and headache.

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The digestive system in humans consists of an alimentary canal and associated digestive glands. The human alimentary canal (aliment: nourish) is a continuous muscular digestive tube about 8 to 10 m long that runs through the body. It is open at two ends with the openings, which are mouth at the anterior end and anus at the posterior end. It performs the function of the digestion of the food. It breaks the food down into smaller substances and absorbs the digested food. The main parts of the alimentary canal are

Alimentary Canal

Mouth and Buccal cavity:

Mouth:

The mouth is the entry point for food. This is the uppermost transverse slit-like opening of the alimentary canal. it is bound by the upper lip and the lower lip. The mouth is used to ingest the food.

Salivary Glands:

Saliva is released by the salivary glands into the oral (buccal) cavity when we smell food. Once the food enters the mouth, chewing (mastication) breaks food into smaller particles it helps enzymes in saliva to attack the broken down food.

Teeth: 

There are total 32 teeth in the buccal cavity of an adult human being. Our teeth can perform a cutting as well as grinding function to accomplish the task of the breaking of the food.

Tongue:

The tongue is a muscular fleshy, triangular-shaped organ which lies along the floor of the buccal cavity. The tongue helps in mixing the food with the saliva and then the tongue and roof of the mouth (soft palate) help to move the food into the pharynx and esophagus. The upper surface of the tongue bears numerous projections called papillae. These papillae contain sensory receptors called taste buds. Taste buds are used to detect tastes of different foods.

Pharynx (or throat):

It is a cavity at the back of the mouth. It is the transition area from the mouth to the esophagus. It is a common passage for the inhaled air and swallowed food. The opening of the respiratory tract in the pharynx is called glottis. It is guarded by a cartilaginous flap called epiglottis. During the act of swallowing the windpipe gets closed by the epiglottis. Thus epiglottis prevents the entry of food particles in the respiratory tract.

Oesophagus:

It is a narrow muscular tube arising from the pharynx, continuing through the thorax and ending in the stomach. It is about 25 cm long. Oesophagus (food pipe) contracts in a synchronized fashion (peristalsis) to move food down towards the stomach. While the muscles behind the food product contract, the muscles ahead of the food relax, causing the forward propulsion of the food. Peristalsis is the main mechanism by which food moves through our digestive system. Mucous secreted by the epithelial cells in the inner lining helps in the smooth passage of food. Once the food approaches the stomach, a muscular valve (esophageal sphincter or cardiac sphincter) relaxes and lets the food pass into the stomach.

Stomach:

The stomach is an elastic bag with highly muscular walls, located below the diaphragm. It can be divided into three parts namely a) Cardiac stomach, b) Fundus stomach, and c) Pyloric stomach. The cardiac stomach is connected with oesophagus and the opening of oesophagus into the cardiac stomach is guarded by the cardiac sphincter. This sphincter prevents the food or acid from the stomach from rentering in the oesophagus. The fundus is the middle part of the stomach. The pyloric stomach is connected with duodenum (small intestine) and the opening of the pyloric stomach into the duodenum is guarded by the pyloric sphincter. This sphincter prevents the food from the duodenum from rentering in the stomach.

The stomach has both a mechanical and a chemical function of digestion. In mechanical function, the upper part of the smooth (involuntary) stomach muscle relaxes to allow a large volume of food to be stored. The lower muscle then contracts in a rhythmical manner in order to churn the food inside and mix it together with the gastric juices. In chemical function, gastric acid (mainly hydrochloric acid) and digestive enzymes Pepsin, Gelatinase and Gastric Amylase and Lipase which break the food further.

At the end of this process, the food is transformed into a thick creamy fluid called chyme. Then the food is pushed into the small intestine.

Small intestine:

It is the longest part of the alimentary canal, a tube about 7 meters long and about 2.5 cm wide. Much coiled and folded, it is contained in the abdomen. It is divided into three parts (i) Duodenum (Short upper part, next to the stomach), (ii) Jejunum (Slightly longer part, about 2 meters long), and (iii) Ileum (Longest, about 4 meters long coiled and twisted).

The chyme from the stomach is then pushed into the duodenum. With the help of enzymes from the pancreas and bile from the liver. A further breakdown of the food occurs in the small intestine. The next two parts of the small intestine (jejunum and ileum) are mostly responsible for the absorption of nutrients from the digested food into the bloodstream through intestinal walls. Then the undigested food is pushed into the large intestine.

Large Intestine:

It is about 1.5 meters long, tube-like organ that is connected to the small intestine at one end and the anus at the other. The large intestine has four parts: cecum, colon, rectum, and anal canal.

• Caecum: It is a small blind pouch at the junction of the small and large intestine. A narrow worm-shaped tube (vermiform appendix) projects from the caecum. The vermiform appendix is a vestigial (functionless) organ in humans but is large and functional in herbivorous mammals for digestion of cellulose.
• Colon: It is a little over a 1-meter long tube, it has three parts termed ascending, transverse and descending limbs of the colon. The colon is lined internally by mucosal cells secreting mucous that makes the passage of undigested material easy. The colon removes water and some nutrients and electrolytes from partially digested food. The remaining material, solid waste called stool, moves through the colon to the rectum.
• Rectum and Anal Canal: It is the last part of the large intestine, about 15 cm. long. It has two parts, the rectum proper and the anal canal. The undigested material called faecal matter is stored in it temporarily.

Anus: 

The anus is the opening at the end of the digestive tract where bowel contents leave the body.  The process is called defaecation or egestion. The anus is surrounded by circular muscles (sphincters). The anal sphincter provides control over releasing stool or holding it. Once stool arrives in the rectum, feedback to the brain makes the person aware of the need for a bowel movement. Voluntary control over the anal sphincter lets us hold the stool until we go to the toilet.

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In this article we shall study, the characteristics of teeth (dentition), different types of teeth and the structure of the tooth.

The teeth are the strongest, hardest and rigid substances in the human body specialized for the biting and grinding of food (known as mastication, or chewing). They form a continuous row in the bottom of the mouth surrounding the tongue on the lateral and anterior sides, as well as another nearly identical row extending from the roof of the mouth. Teeth form deep roots into the bones of the maxillae and the mandible but grow out through the gums of the mouth to form biting surfaces. Normal adults have 32 teeth, which are distributed in two dental arches. One is called the maxillary arch and the other is called the mandibular arch.

Man is provided with two sets of teeth, which make their appearance at different periods of life. Those of the first set appears in childhood and are called the deciduous or milk teeth. Those of the second set, which also appear at an early period, may continue until old age and are named permanent. The deciduous teeth are twenty in number: four incisors, two canines, and four molars, in each jaw. The permanent teeth are thirty-two in number: four incisors, two canines, four premolars, and six molars, in each jaw. The third molars are called wisdom teeth because they usually appear in a person’s late teens or early twenties when the person is old enough to have acquired some wisdom.

Characteristics of Teeth:

The Condont Dentition:

It is an arrangement in which the base of the tooth is completely enclosed in a deep socket of bone is the characteristic of mammalian teeth as seen in humans, loins, goats, etc. this arrangement makes the attachment strongest in the vertebrates.  Such teeth are embedded in pits of maxillae in upper jaw and mandibles in the lower jaw. These pits are called sockets or alveoli.  The animals having a condont dentition cannot lose their teeth easily as compared to Acrodont, where teeth are attached on the top surface of the jaw bone as in fish and amphibians. Acrodont type of attachment is not very strong and teeth are lost easily and are replaced by new ones.

Diphyodont Dentition:

Human teeth are diphyodont because these are formed in two sets. It is a characteristic of mammals in which milk teeth (first set) appear in the young ones but as they grow and jaw becomes larger, milk teeth are replaced by larger permanent ones (second set) to fit in the larger jaw bone.

Primary Dentition:

There are  20 primary or deciduous teeth in children. In general,  the order of eruption is two  lower central incisors, 6 to 8 months; two upper central incisors, 5 to 7 months; two lower lateral incisors, 8 to 11 months; two upper lateral incisors, 7 to 10 months; four canines (cuspids), lower and upper, 16 to 20 months; four first molars, lower and upper, 10 to 16 months; four second molars, upper and lower, 20 to 30 months. Thus eruption of primary sets of teeth is completed by the age of 2.5 years.

The dental formula which is a representation of the number of teeth in either half of upper and lower jaws for primary dentition is

i 2/2, c 1/1, pm 0/0, m 2/2

i.e. total (2 + 2 + 1 + 1 + 0 + 0 + 2 + 2 = 10) x 2 halves = 20 teeth

Secondary Dentition:

The secondary or permanent teeth, which begin to erupt at about 6 years of age in people. These are completed by the 16th year with the exception of third molars, which appear between the 18th and 25th years. In some individuals the third molars, although present beneath the gingiva, do not erupt.  There are 32 teeth in the permanent set. The dental formula for secondary dentition is

i 2/2, c 1/1, pm 2/2, m 3/3

i.e. total (2 + 2 + 1 + 1 + 2 + 2 + 3 + 3 = 16) x 2 halves = 32 teeth

Heterodont dentition:

Due to Heterodont dentition, humans have different shapes of teeth to carry different functions.  There are 4 functionally different types of teeth, namely, flat incisors, canines, premolars, and molars. Pre-molars have no counterparts in the milk teeth.

Incisors:

Incisors are front teeth, which are flat and chisel-shaped. These have sharp cutting edges for cutting, chopping, and gnawing. They have a single root. In an elephant, incisors grow out into tusks and are not used for biting or cutting.

Canines:

Canines are more pointed and cone-shaped or dagger-shaped. They have a single root. They are used for ripping and sheading. In carnivorous animals, they are long and prominent. They are usually absent in herbivorous animals. In walrus upper canines grow out into tusks.

Premolars and Molars:

Premolars and molars are broad and have flat surfaces. These are used for shearing, crushing and grinding. They are also called grinding teeth.  They have two roots. There are third molars called wisdom teeth. Some persons do not get them or get removed for a faulty eruption.  In herbivorous animals, premolars and molars are broad and high cusps for eating and chewing grass. In carnivorous they have sharp cusps for shearing flesh.

Structure of a tooth:

The tooth is placed in a socket or alveolus (plural: alveoli) over the jaw bone. It is distinguished into three parts: root, Neck, and Crown. Each tooth consists of a crown with one or more tips (cusps), a neck, and a root. The clinical crown is that part of the tooth exposed in the oral cavity. The anatomical crown is the entire enamel-covered part of the tooth.

Root:

The root is the basal part of the tooth that extends into the bone and holds the tooth in place. It makes up approximately two-thirds of the tooth. It’s made up of following parts:

• Root canal: The root canal is a passageway that contains pulp. It provides a passageway for the nerves and blood vessels.
• Cementum: it is also called cement. It is a hard connective tissue that covers a tooth’s root. It is connected to the periodontal ligament.
• Periodontal ligament: It is made up of collagenous connective tissue fibers by which a tooth’s root is connected to its socket. It contains both nerves and blood vessels. Along with the cementum.
• Nerves and blood vessels: Blood vessels supply the periodontal ligament with nutrients, while nerves help control the amount of force used when you chew.
• Jaw bone. The jaw bone is also called the alveolar bone. It encompasses the roots of the teeth and holds the teeth in place.

Neck:

It is also called the dental cervix, which sits between the crown and root. It is part above the root and enclosed in a soft, fleshy gum or gingiva. It forms the line where the cementum (that covers the root) meets the enamel. It has three main parts:

• Gums: They are also called gingiva, are the fleshy, pink connective tissue that’s attached to the neck of the tooth and the cementum. Teeth are housed in gums. Gums protect a tooth’s roots and any teeth that have yet to erupt.
• Pulp: The pulp is the innermost portion of the tooth that houses the blood vessels, nerves, and connective tissue. This region is sensitive to pain. The nerves and blood vessels of the tooth enter and exit the pulp through a hole at the point of each root called the apical foramen.
• Pulp cavity: The pulp cavity, sometimes called the pulp chamber, is a central space bounded by a layer of odontoblast cells and filled with soft pulp.

Crown:

This is the top, visible part of a tooth. It contains three parts:

• Anatomical crown. This is the top portion of a tooth. It’s usually the only part of a tooth that we can see.
• Enamel: This is the outermost layer of a tooth. It covers and protects all teeth. It is the hardest substance in the human body. It helps to protect teeth from bacteria. It also provides strength so the teeth can withstand pressure from chewing. Since it doesn’t regenerate, preventing tooth decay is critical to keep the enamel intact.
• Dentin: This layer is located beneath the enamel and cementum. Chemically it is similar to bone. It extends from the crown down through the neck and root. Microscopic tubules that allow various food types access to nerves are enclosed here. It protects teeth from heat and cold. When enamel is worn away from a tooth, the dentin becomes vulnerable to sensitivity.

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Science > Biology > Digestion and absorption in Human > Buccal cavity – Teeth

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The digestive system in humans consists of an alimentary canal and associated digestive glands. The human alimentary canal (aliment: nourish) is a continuous muscular digestive tube about 8 to 10 m long that runs through the body. It is open at two ends with the openings, which are mouth at the anterior end and anus at the posterior end. It performs the function of the digestion of the food. It breaks the food down into smaller substances and absorbs the digested food.  In this article, we shall study the mouth and the buccal cavity.

Lips (labia oris):

The lips are soft, fleshy structures that form the anterior border of the external opening of the mouth. The lips are very flexible and elastic structures and contain many collagen and elastin fibers and adipose tissue covered by a thin layer of stratified squamous epithelium. The exterior of the lips is continuous with the skin and is covered by keratinized epithelium, while the inner surface is continuous with the mucous membrane of the mouth and is covered by non-keratinized epithelium. The inner surface of each lip is connected in the middle line to the corresponding gum by a fold of mucous membrane, the frenulum—the upper being the larger. The color from the underlying blood vessels can be seen through the relatively transparent epithelium tissues, giving the lips a reddish-pink to dark red appearance, depending on the overlying pigment.

Cheeks (buccae):

They are fleshy structures which form the sides of the face and are continuous in front with the lips. Similar to the lips, the exterior of the cheeks is covered in keratinized stratified squamous epithelium continuous with the skin and the interior is covered in nonkeratinized stratified squamous epithelium continuous with the mucous membrane. Between the epithelium, layers are layers of connective tissues, nerves, and muscles. In particular, the muscles of the cheeks include the buccinator, orbicularis oris and zygomaticus major, which move the lips and cheeks.

The lips and cheeks are important in the processes of mastication and speech. They help manipulate food within the mouth and hold it in place while the teeth crush or tear it.

Mouth:

The mouth is the entry point for food. This is the uppermost transverse slit-like opening of the alimentary canal. It is bound by the upper lip and the lower lip. The mouth is used to ingest the food. The mouth leads into the vestibule.

Oral cavity:

The mouth cavity is divided into two sections: The first section is called Vestibule (vestibulum oris) and the second section is the Oral Cavity Proper (cavum oris proprium).

Vestibule:

It is narrow space enclosed between the lips and cheeks externally and a pair of jaws internally, Its lining contains mucous glands. The vestibules lead into the oral cavity proper.

Oral cavity Proper:

The mouth, is bounded by the lips anteriorly, the fauces posteriorly, the cheeks laterally. It is bounded laterally and in front by the alveolar arches with their contained teeth; behind, it communicates with the pharynx. The upper jaw is fixed, which forms the roof of the mouth cavity and consists of palate, teeth, and gums surrounding the teeth. The lower jaw is movable and forms the floor of the mouth cavity, which consists of the tongue along with the teeth and gums surrounding them. The oral cavity is lined with moist stratified squamous epithelium, which provides protection against abrasion.

The main parts of the buccal cavity or an oral cavity are as follows:

Palate:

In humans, the buccal cavity and the nasal cavity are separated by the palate. Palate forms the roof of the oral or buccal cavity. The palate consists of two parts, an anterior bony part, the hard palate, and a posterior, non-bony part, the soft palate, which consists of skeletal muscle and connective tissue. Hard palate is supported by bones, Its mucous membrane bears transverse ridges called rugae. Which keep the food in place during mastication. The smooth-surfaced soft palate makes swallowing easy. The posterior end of soft palate hangs down as a small flap called the uvula or velum palati. During swallowing process, the uvula prevents food from passing into the nasal cavity by closing the internal nares. Palatine tonsils are located in the lateral wall of the fauces.

Tongue

The tongue is a muscular fleshy, triangular shape, highly mobile organ which lies along the floor of the buccal cavity. It is a large, muscular organ, which occupies most of the oral cavity and can take up a variety of shapes and positions. it is actually an organ made of epithelium, several skeletal muscles, nerves, and connective tissues. The anterior part of the tongue is relatively free and is attached to the floor of the mouth by a thin fold of tissue called the frenulum linguae. The intrinsic muscles in tongue are largely responsible for changing the shape of the tongue, such as flattening and elevating the tongue during drinking and swallowing. The extrinsic tongue muscles protrude and retract the tongue, move it from side to side, and change its shape. Mucous membrane secrets mucus that keeps the tongue moist.

Papillae on Tongue:

The tongue contains many small ridges known as lingual papillae that help it to grip and move food around the mouth. There are three types of papillae.

• Vallate Paillae: They are 8 to 12 in numbers and are arranged in an inverted V at the posterior part of tongue. Each papilla bears up to 100 taste buds.
• Fungiform Papillae: They are mushroom-shaped and are more numerous near the tip of the tongue. Each papilla bears about 5 taste buds.
• Filiform Papillae: They are smallest papillae and are thread-like. They are distributed on 2/3^rd portion of anterior tongue. They do not have taste buds. They are tactile receptors.

Taste Buds:

Taste buds are hidden in valleys around some of the papillae and produce the sense of taste by detecting chemicals found in food. The taste buds located at the tip of the tongue, taste for sweets. Those presents on sides, taste for sour and salt. Those located at the posterior part of the tongue are meant for bitter taste.

The major functions of the tongue are the sense of Taste, organ of speech, oral cleansing of the mouth, holding the food in place during mastication, and Squeezing food into the oropharynx while swallowing.

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