Mandibular first premolar
The mandibular first premolar is the tooth located laterally from both the mandibular canines of the mouth but mesial from both mandibular second premolars. The function of this premolar is similar to that of canines in regard to tearing being the principal action during mastication known as chewing. Mandibular first premolars have two cusps; the one large and sharp is located on the buccal side of the tooth. Since the lingual cusp is small and nonfunctional, the mandibular first premolar resembles a small canine. There are no deciduous mandibular premolars. Instead, the teeth that precede the permanent mandibular premolars are the deciduous mandibular molars. Sometimes, premolars are referred to as bicuspids. Though the terms are synonymous, "bicuspid" refers to having two functional cusps, the mandibular first premolar is an example of a premolar with only one functional cusp. Thus, "bicuspid" is technically not as accurate as "premolar". In the universal system of notation, the permanent mandibular premolars are designated by a number.
The right permanent mandibular first premolar is known as "28", the left one is known as "21". In the Palmer notation, a number is used in conjunction with a symbol designating in which quadrant the tooth is found. For this tooth, the left and right first premolars would have the same number, "4", but the right one would have the symbol, "┐", over it, while the left one would have, "┌"; the international notation has a different numbering system than the previous two, the right permanent mandibular first premolar is known as "44", the left one is known as "34". Ash, Major M. and Stanley J. Nelson, 2003. Wheeler’s Dental Anatomy and Occlusion. 8th edition
Apes are a branch of Old World tailless simians native to Africa and Southeast Asia. They are the sister group of the Old World monkeys, they are distinguished from other primates by a wider degree of freedom of motion at the shoulder joint as evolved by the influence of brachiation. In traditional and non-scientific use, the term "ape" excludes humans, is thus not equivalent to the scientific taxon Hominoidea. There are two extant branches of the superfamily Hominoidea: lesser apes; the family Hylobatidae, the lesser apes, include four genera and a total of sixteen species of gibbon, including the lar gibbon and the siamang, all native to Asia. They are arboreal and bipedal on the ground, they have lighter smaller social groups than great apes. The family Hominidae, the great apes, includes three extant species of orangutans and their subspecies, two extant species of gorillas and their subspecies, two extant species of chimpanzees and their subspecies, one extant species of humans in a single extant subspecies.
Except for gorillas and humans, hominoids are agile climbers of trees. Apes eat a variety of plant and animal foods, with the majority of food being plant foods, which can include fruit, stalks and seeds, including nuts and grass seeds. Human diets are sometimes different to that of other apes due in part to the development of technology and a wide range of habitation. Humans are by far the most numerous of the ape species, in fact outnumbering all other primates by a factor of several thousand to one. Most non-human hominoids are rare or endangered; the chief threat to most of the endangered species is loss of tropical rainforest habitat, though some populations are further imperiled by hunting for bushmeat. The great apes of Africa are facing threat from the Ebola virus. Considered to be the greatest threat to survival of African apes, Ebola is responsible for the death of at least one third of gorillas and chimpanzees since 1990. "Ape", from Old English apa, is a word of uncertain origin. The term has a history of rather imprecise usage -- and of punning usage in the vernacular.
Its earliest meaning was of any non-human anthropoid primate, as is still the case for its cognates in other Germanic languages. After the term "monkey" had been introduced into English, "ape" was specialized to refer to a tailless primate. Thus, the term "ape" obtained two different meanings, as shown in the 1911 Encyclopædia Britannica entry: it could be used as a synonym for "monkey" and it could denote the tailless humanlike primate in particular. Some, or all, hominoids are called "apes", but the term is used broadly and has several different senses within both popular and scientific settings. "Ape" has been used as a synonym for "monkey" or for naming any primate with a human-like appearance those without a tail. Biologists have traditionally used the term "ape" to mean a member of the superfamily Hominoidea other than humans, but more to mean all members of Hominoidea. So "ape"—not to be confused with "great ape"—now becomes another word for hominoid including humans; the term hominoid is not to be confused with the family of great apes.
The distinction between apes and monkeys is complicated by the traditional paraphyly of monkeys: Apes emerged as a sister group of Old World Monkeys in the catarhines, which are a sister group of New World Monkeys. Therefore, apes and related contemporary extinct groups such as Parapithecidaea are monkeys as well, for any consistent definition of "monkey". "Old World Monkey" may legitimately be taken to be meant to include all the catarrhines, including apes and extinct species such as Aegyptopithecus, in which case the apes and Aegyptopithecus emerged within the Old World Monkeys. The primates called "apes" today became known to Europeans after the 18th century; as zoological knowledge developed, it became clear that taillessness occurred in a number of different and otherwise distantly related species. Sir Wilfrid Le Gros Clark was one of those primatologists who developed the idea that there were trends in primate evolution and that the extant members of the order could be arranged in an "..
Ascending series", leading from "monkeys" to "apes" to humans. Within this tradition "ape" came to refer to all members of the superfamily Hominoidea except humans; as such, this use of "apes" represented a paraphyletic grouping, meaning that though all species of apes were descended from a common ancestor, this grouping did not include all the descendant species, because humans were excluded from being among the apes. The cladogram of the superfamily Hominoidae shows the descendant relationships of the extant hominoids that are broadly accepted today. For each clade, it is indicated how many million of years ago newer extant clades radiated. Traditionally, humans were considered neither apes nor great apes, but today they are recognized as having emerged deep in the phylogenetic tree of apes. Thus, there are at least three common, or traditional, uses of the term "ape": non-specialists may not distinguish between "monkeys" and "apes", that is, they may use the two terms interchangeably. Modern biologists and primatologists use monophyletic groups for taxonomic classification.
Incisors are the front teeth present in most mammals. They are located in the premaxilla on the mandible below. Humans have a total of eight. Opossums have 18. Adult humans have eight incisors, two of each type; the types of incisor are: maxillary central incisor maxillary lateral incisor mandibular central incisor mandibular lateral incisor Children with a full set of deciduous teeth have eight incisors, named the same way as in permanent teeth. Young children may have from zero to eight incisors depending on the stage of their tooth eruption and tooth development; the mandibular central incisors erupt first, followed by the maxillary central incisors, the mandibular lateral incisors and the maxillary laterals. The rest of the primary dentition erupts after the incisors. Apart from the first molars, the incisors are the first permanent teeth to erupt, following the same order as the primary teeth, among themselves. Among other animals, the number varies from species to species. Opossums have 18.
Cats, foxes and horses have twelve. Rodents have four. Rabbits and hares were once considered rodents, but are distinguished by having six—one small pair, called "peg teeth", is located directly behind the most anterior pair. Incisors are used to bite off tough foods, such as red meat. Cattle a total of six on the bottom. In cats, the incisors are small. In elephants, the upper incisors are modified into curved tusks; the incisors of rodents are worn by gnawing. In humans, the incisors serve to cut off pieces of food, as well as in the grip of other food items. Canine tooth Molar Premolar Shovel-shaped incisors Media related to Incisors at Wikimedia Commons
The molars or molar teeth are large, flat teeth at the back of the mouth. They are more developed in mammals, they are used to grind food during chewing. The name molar derives from Latin, molaris dens, meaning "millstone tooth", from mola and dens, tooth. Molars show a great deal of diversity in shape across mammal groups. In humans, the molar teeth have either five cusps. Adult humans have 12 molars, in four groups of three at the back of the mouth; the third, rearmost molar in each group is called a wisdom tooth. It is the last tooth to appear, breaking through the front of the gum at about the age of 20, although this varies from individual to individual. Race can affect the age at which this occurs, with statistical variations between groups. In some cases, it may not erupt at all; the human mouth contains lower molars. They are: maxillary first molar, maxillary second molar, maxillary third molar, mandibular first molar, mandibular second molar, mandibular third molar. In mammals, the crown of the molars and premolars is folded into a wide range of complex shapes.
The basic elements of the crown are the more or less conical projections called cusps and the valleys that separate them. The cusps contain both dentine and enamel, whereas minor projections on the crown, called crenullations, are the result of different enamel thickness. Cusps are joined to form ridges and expanded to form crests. Cingula are incomplete ridges that pass around the base of the crown; these mammalian, multicusped cheek teeth evolved from single-cusped teeth in reptilians, although the diversity of therapsid molar patterns and the complexity in the molars of the earliest mammals make determining how this happened impossible. According to the accepted "differentiation theory", additional cusps have arisen by budding or outgrowth from the crown, while the rivalling "concrescence theory" instead proposes that complex teeth evolved by the clustering of separate conical teeth. Therian mammals are agreed to have evolved from an ancestor with tribosphenic cheek teeth, with three main cusps arranged in a triangle.
Each major cusp on an upper molar is called a cone and is identified by a prefix dependent on its relative location on the tooth: proto-, para-, meta-, hypo-, ento-. Suffixes are added to these names: -id is added to cusps on a lower molar. A shelf-like ridge on the lower part of the crown is called a cingulum; the design, considered one of the most important characteristics of mammals is a three-cusped shape called a tribosphenic molar. This molar design has two important features: the trigonid, or shearing end, the talonid, or crushing heel. In modern tribosphenic molars, the trigonid is towards the front of the jaw and the talonid is towards the rear; the tribosphenic tooth is found in young platypuses. Upper molars look like three-pointed mountain ranges; the tribosphenic design appears primitively in all groups of mammals. Some paleontologists believe that it developed independently in monotremes, rather than being inherited from an ancestor that they share with marsupials and placentals. For example, the dentition of the Early Cretaceous monotreme Steropodon is similar to those of Peramus and dryolestoids, which suggests that monotremes are related to some pre-tribosphenic therian mammals, but, on the other hand, the status of neither of these two groups is well-established.
Some Jurassic mammals, such as Shuotherium and Pseudotribos, have "reversed tribosphenic" molars, in which the talonid is towards the front. This variant is regarded as an example of convergent evolution. From the primitive tribosphenic tooth, molars have diversified into several unique morphologies. In many groups, a fourth cusp, the hypocone, subsequently evolved. Quadrate molars have an additional fourth cusp on the lingual side called the hypocone, located posterior to the protocone. Quadrate molars appeared early in mammal evolution and are present in many species, including hedgehogs and many primates, including humans. There may be a fifth cusp. In many mammals, additional smaller cusps called, they are named after their locations, e.g. a paraconule is located between a paracone and a metacone, a hypoconulid is located between a hypoconid and an entoconid. In bunodont molars, the cusps rounded hills rather than sharp peaks, they are most common among omnivores such as pigs and humans. Bunodont molars are effective crushing devices and basically quadrate in shape.
Hypsodont dentition is characterized by high-crowned teeth and enamel that extends far past the gum line, which provides extra material for wear and tear. Some examples of animals with hypsodont dentition are cattle and horses, all animals that feed on gritty, fibrous material. Hypsodont molars can continue to grow for example in some species of Arvicolinae. Hypsodont molars lack both a neck; the occlusal surface is rough and flat, adapted for crushing and grinding plant material. The body is covered with cementum both above and below the gingival line, below, a layer of enamel covering the entire length of the body; the cementum and the enamel invaginate into the thick
Mandibular second premolar
The mandibular second premolar is the tooth located distally from both the mandibular first premolars of the mouth but mesial from both mandibular first molars. The function of this premolar is assist the mandibular first molar during mastication known as chewing. Mandibular second premolars have three cusps. There is one large cusp on the buccal side of the tooth; the lingual cusps are well functional. Therefore, whereas the mandibular first premolar resembles a small canine, the mandibular second premolar is more alike to the first molar. There are no deciduous mandibular premolars. Instead, the teeth that precede the permanent mandibular premolars are the deciduous mandibular molars. Anatomy: The mandibular second premolar most has three cusps but can have two as well; the three cusp variety has one large cusp on the buccal with two smaller lingual cusps. The mesiolingual cusp is twice the size of the distolingual cusp. Viewed from the occlusal the tooth is rather square in outline on the lingual.
The occlusal table is rectangular. The groove pattern is shaped like a “Y” with the tail pointed to the lingual and placed between the distolingual and mesiolingual cusps one third of the distance form the distal to the mesial; the contacts with the adjacent teeth are positioned buccal to the midpoint. Viewed from the buccal the buccal cusp tip is centered mesiodistally; the buccal cusp ridges exhibit slight concavities that extend over the buccal surfaces as developmental grooves into the gingival embrasure. The contacts with adjacent teeth are in the occlusal third of the tooth with the distal height of contour closer to the gingival than the mesial height of contour; the root is straight with slight curvature to the distal in the apical third. Viewed from the mesial or distal the buccal height of contour is in the gingival third of the tooth; the lingual height of contour is in the middle third of the tooth. When divided into thirds from the buccal height of contour to the lingual height of contour, the buccal cusp is at the contact between the buccal and middle thirds and the central groove is at the contact of the middle and lingual thirds.
The two cusp variety has a groove pattern shaped like a “U” or “H”. Viewed from the occlusal it is more rounded in general and its lingual cusp is positioned to the mesial, while the occlusal table remains squared. Viewed from the buccal the buccal cusp is centered over the root as in the three cusp variety. Viewed from the mesial or distal its heights of contour are similar to the three cusp variety. Sometimes, premolars are referred to as bicuspids. Though the terms are synonymous, "bicuspid" refers to having two functional cusps, the mandibular second premolar is an example of a premolar with three functional cusps. Thus, "biscupid" is technically not as accurate as "premolar". In the universal system of notation, the permanent mandibular premolars are designated by a number; the right permanent mandibular second premolar is known as "29", the left one is known as "20". In the Palmer notation, a number is used in conjunction with a symbol designating in which quadrant the tooth is found. For this tooth, the left and right second premolars would have the same number, "5", but the right one would have the symbol, "┐", over it, while the left one would have, "┌".
The international notation has a different numbering system than the previous two, the right permanent mandibular second premolar is known as "45", the left one is known as "35". It is a common condition in orthodontics for a patient to have one or both mandibular second premolars congenitally absent. Ash, Major M. and Stanley J. Nelson, 2003. Wheeler’s Dental Anatomy and Occlusion. 8th edition