The '1' in the 2:1:2:3 dental formula of a modern human adult refers to canine teeth. There is one canine tooth, situated between the incisors and premolars, in each of the four quadrants of the modern human adult dental arcade, for a total of four canine teeth per individual.

The second '2' in the 2:1:2:3 dental formula of a modern human adult refers to premolars. There are two premolars, situated between the canine and molars, in each of the four quadrants in a modern human adult, for a total of eight of this type of tooth per individual. Primitive mammals have four pre-molars; in the line of evolution leading to humans, the first two premolars were lost, leaving the third and fourth.

The '3' in the 2:1:2:3 dental formula of a modern human adult refers to molars: first, second, and third, or wisdom tooth (even though the wisdom tooth is indeed a molar, it is only one of three molars per quadrant). In some individuals, this third molar is not present in some or all quadrants. These teeth are situated furthest back in the dental arcade, behind the premolars.

The coronal (frontal) suture runs laterally from the top of the cranium and ties together the frontal and two parietal bones.

The sagittal suture runs along the top of the cranium, between the two parietal bones. The lambdoidal suture ties together the occipital and two parietal bones on the posterior cranium. The squamosal suture is the area where the squamosal section of the temporal bone is beveled and overlaps the parietal bone. The basilar suture, also referred to as spheno-occipital synchondrosis, is a suture between the occipital and sphenoid bones.

The squamosal suture is the area where the squamosal section of the temporal bone is beveled and overlaps the parietal bone.

The sagittal suture runs along the top of the cranium, between the two parietal bones. The coronal suture runs laterally from the top of the cranium and ties together the frontal and two parietal bones. The lambdoidal suture ties together the occipital and two parietal bones on the posterior cranium. The basilar suture, also referred to as spheno-occipital synchondrosis, is a suture between the occipital and sphenoid bones.

The sagittal suture is the long suture that runs between the two parietal bones, running antero-posteriorally along the top of the cranium.

The coronal suture runs laterally from the top of the cranium and ties together the frontal and two parietal bones. The lambdoidal suture ties together the occipital and two parietal bones on the posterior cranium. The squamosal suture is the area where the squamosal section of the temporal bone is beveled and overlaps the parietal bone. The basilar suture, also referred to as spheno-occipital synchondrosis, is a suture between the occipital and sphenoid bones.

The optic canal is also referred to as the optic foramen. It is a short canal through the lesser wing of the sphenoid bone. The optic canal transmits the optic nerve (cranial nerve II) and the ophthalmic artery into the orbital cavity. 

The three branches of the trigeminal nerve, the ophthalmic, maxillary, and mandibular branches leave the skull through three separate foramina. The opthalmic branch exits through the superior orbital fissure, the maxillary branch exists through the foramen rotundum, and the mandibular branch exits through the foramen ovale. 

The spinal cord, accessory nerve, and various arteries pass through the foramen magnum. The cribriform plate allows the emergence of the olfactory nerve. The obturator foramen is in the hip.

Humans skulls contain sutures, which are dense fibrous connective tissue joints that holds the bony plates of the skull together. The lambdoid suture connects the parietal bones with the occipital bone. This suture is actually continuous with the occipitomastoid suture.

The skeletal system is divided into the axial skeleton and the appendicular skeleton. The axial skeleton consists of bones of the head, ribs, vertebral column, and sternum. In contrast, the appendicular skeleton consists of bones of the extremities, including the fingers, arms, and legs.