Section 4-4. Teeth as the fourth special sensory organ which is proved by the evolution history

According to current anthropology and medical science, specific sensory organs located in our head region include three organs such as eyes for visual sense, nose for olfactory sense and ears for auditory sense・equilibratory sense. However, we think that it is necessary to reconsider it. As stated above, in the central dental pulp, the nerve entering from apical foramen forms nerve plexus with a part of it invading into the inside of dentinum. In addition, there is periodontal membrane with well-developed nerves and blood vessels around the dental root. The nerves observed in the periodontal membrane are not so simple as those found in usual tissues like skin but form both free nerve ending as the mesh-like algesiroreceptor, and tactile perception. Via these nervous tissues, we perceive the physical characteristics of the foods, including not only cold・warm sensation but also “firmness” by using teeth as the sensory organs. Promptly, such information is transmitted to the brain where judgment is made whether the foods in question should be ingested or not, or instruction is provided to secrete saliva containing digestive enzymes, and furthermore, digestive organs including stomach and intestines are instructed to make preparation of their functioning. Thus, teeth play a role as the gatekeeper for the food inlet and the sensory organ to control tactile perception and cold・warm sensation.
Presence of such teeth as the sensory organs can be traced back in the evolution process. “Jawless vertebrates” as the oldest vertebrate which lived at the early part of Palaeozoic Era (500 million years ago) had neither jaws nor teeth; however, in place of them, they girded themselves with lots of carapace known as “armor”. On the surface of the armor, there were many small pieces of dentinal matrix which is fundamentally the same as those in human teeth. They are morphologically the tissues with projected cells which contribute to be the sensory organs to detect changes in outer circumstances.
Three sensory organs such as eyes, nose and ears are common in all of vertebrates. They might be produced during the course of development to vertebrates; however, evolution of teeth was much more delayed. In the midst of Palaeozoic Era (400 million years ago), ancient sharks having jaws and teeth newly appeared. One part of branchial cartilage became jaws which could work as the active switching device while the protrusion of dentinal matrix above it was changed to teeth to catch the food. In contrast, however, sharks have different structure with teeth being not directly integrated with maxillary cartilage (called as “Fiber bonding”). When evolution proceeded to bony fishes, teeth directly bound jawbone to be firmly supported on the jaw. Around 360 million years ago, one of the bony fishes succeeded in evolution to the oldest amphibian which could spend their lives on shore. This amphibian had the similar epithelial dental enamel as those in humans. This is because teeth of them were exposed to the air on land, whereby the superficial “Enameloid” of teeth which corresponded to the dentinal matrix of fishes underwent evolution.
In the latter part of Pareozoic Era (300 million years ago), reptiles which completely acclimatized to the on-shore lives appeared while in the early part of the Mesozoic Era (220 million years ago), their primitive groups evolved into mammals. Teeth achieved great changes during the course of their evolution processes. In reptiles, their teeth have conical shapes and they swallow the foods without chewing. In mammals, however, their teeth are differentiated into incisor, canine, bicuspid and molar teeth while in particular, the molar teeth take “Tribosphen type” by which both upper and lower molars cooperate each other in pairwise association to cut and mash up the foods. This cooperative mechanism is responsible for slow chewing in the mouth and appropriate digestion. Furthermore, the teeth of mammals were equipped with periodontal membrane surrounding the dental root to permit strong and functional conjunction with gnathic bone.
As stated above, this periodontal membrane possesses extremely sophisticated sensory receptor which can be compared to “Listening with teeth”, if we employ the expression such as “Looking with eyes”, ”Sniffing with nose”, and “Hearing with ears”. The teeth of mammarians are changed only once in the lives. The dental enamel becomes thicker, with their crystal and trabecular architecture being developed. Thus, upon appearance of mammalians, revolutionary or completely heterogenous teeth appeared after highly significant evolution. As briefed above, in the early stage of vertebrate evolution, brain looked like a fistulae containing nerve cells. During the course of evolution to human beings, the brain formed so much complicated structure consisting of cerebrum, diencephalons, mesencephalon, cerebellum, medullary and spinal cord; as the natural consequence, the brain was developed to control not only vital activities of individuals but also sublime mental activities. The central nerves in the brain are operated by the interconnection networks comprising nerve cells (neuron), but the network of the neuron was not genetically decided from the very beginning. They were prepared by stimulation of sensory information which was inputted during the course of growth. As is obvious from the above explanation, the brain and the central nervous system are developed and grown in response to peripheral stimulation. This reminds us of the opinion that during our childhood, appropriate stimulation is indispensable for growth of the brain. Given all this, it is conceivable that the brain of vertebrates was developed by increased stimulation due to lots of sensory information from peripheral areas to the brain through three specific sensory organs such as eyes, nose and ears. Among these vertebrates, mammals could be considered to achieve further evolution from the brain of the reptile levels thanks to appearance of the fourth specific new sensory organ, namely “teeth”. Accordingly, the current status of mammals and human beings could be attributed to presence of “teeth”. In this sense, we can say that the brain and the central nervous system of human beings are closely correlated to “teeth” and “Hazo”, whereby it goes without saying that abnormality of teeth and Hazo induces dysfunction of them, leading to onset of systemic dysfunction or psychosomatic abnormalities.
Thus, teeth should be positioned as the fourth sensory organ, following eyes, nose and ears as the conventional sensory organs. Throughout all the life stages from babies, children, adults and the elderly persons, their roles which participated in the evolution processes contribute to growth and development of the brain, improvement of their functions and maintenance.