usually form into pairs and then fuse together. As the cranium fuses, sutures are formed that resemble stitching between bone plates. In a newborn, the junction of the parietal bones with the frontal and occipital bones, form the anterior (front) and posterior (back) fontanelle, or soft spots. The separation of the cranial bone plates at time of birth facilitate passage of the head of the fetus through the mother's birth canal, or pelvic girdle. The parietal bones, and occipital bone can overlap each other in the birth canal, and form the unusual looking "cone head" appearance in a newborn when delivered in a natural, or vaginal, delivery. Teeth Humans normally will produce two sets of teeth called primary dentition, or deciduous teeth, and secondary dentition, or permanent teeth. A tooth is the toughest known substance in the body exceeding bones in density and strength. Tooth enamel lends great strength to the tooth structure. The formation of a developing tooth includes the process of dentin formation, (see: Dentinogenesis) and enamel formation, (see: amelogenesis). The tooth breaks through the gum into the mouth in a process called eruption. The formation of teeth begins in early fetal development and goes through six stages: (1) initiation stage, 6th - 7th week (2) bud stage, 8th wk (3) cap stage, 9th-10 wk (4) bell stage, 11th-12th wk (5) apposition (6) maturation stage Clinical significance Infection Severe viral infections that affect the mouth, lips, or the oral cavity include: Oral cancer may have a viral link. Minor viral infections include: Mumps is a viral infection of the parotid salivary glands. Chicken pox is a viral infection that can spread to the mouth. Thrush (Candidiasis) fungal infection. Tonsillitis is an inflammation of the tonsils and may cause a sore throat and fever. In chronic cases tonsillectomy may be required. Infected teeth can on rare occasions cause infection to spread leading to cavernous sinus thrombosis, mediastinitis, or Ludwig's angina causing airway blockage. Diseases may be transmitted by contact of the head, mouth, or body fluids, such as Herpes Simplex Virus Type I (HSV-1), Herpes Simplex Virus Type II (HSV-2) genital herpes, which may present as a lesion on the lips, and contactable via skin to skin contact Other Other diseases include: Gingivitis gum disease, periodontal disease, oral forms of syphilis and gonorrhea. Dental caries or dental cavities. Temporomandibular joint diseases and disorders, commonly called TMJ. Autoimmune diseases such