The human skull, a complex bony structure, primarily serves to protect the delicate brain and support the sensory organs of the face. While it appears as a single, rigid unit in adults, the concept of a “softest part” typically refers to specific, flexible areas present in infants. These unique features accommodate rapid growth and facilitate passage through the birth canal, differentiating the developing skull from its mature, fused counterpart.
The Unique Soft Spots of an Infant Skull
Infant skulls feature several “soft spots” known as fontanelles, which are gaps of membranous connective tissue located between the cranial bones. A newborn’s skull is composed of multiple bones, including two frontal bones, two parietal bones, and one occipital bone, which are not yet fully fused. Instead, they are joined by fibrous sutures, allowing for movement and flexibility.
There are typically six fontanelles present at birth, with the two most prominent and commonly known being the anterior and posterior fontanelles. The anterior fontanelle, often referred to as the main soft spot, is diamond-shaped and located on the top-front of the head, where the frontal and parietal bones meet. The posterior fontanelle is triangular and found at the back of the head, where the two parietal bones join the occipital bone. Additionally, there are two mastoid fontanelles and two sphenoidal fontanelles, located on the sides of the skull, though these typically close much sooner after birth.
The timing of fontanelle closure varies. The posterior fontanelle usually closes within 6 to 8 weeks after birth. The anterior fontanelle remains open for a longer period, typically closing between 7 and 19 months. This gradual closure occurs as the fibrous connective tissue ossifies, knitting the skull bones together.
The Purpose of Infant Soft Spots
The presence of fontanelles serves important biological functions for a developing infant. One primary purpose is to facilitate childbirth. During passage through the narrow birth canal, the flexible nature of the fontanelles and sutures allows the bony plates of the skull to overlap and compress. This process, known as molding, helps the baby’s head navigate the birth canal more easily and reduces the risk of injury to both the baby and the mother.
Beyond birth, fontanelles are important for accommodating the rapid growth of the infant’s brain. An infant’s brain undergoes significant development in the first few years of life, expanding faster than the surrounding bone can grow. The flexible sutures and membranous fontanelles provide the necessary space for the brain to grow and develop without constraint. Without this flexibility, the brain would not be able to expand adequately, potentially leading to developmental issues.
Fontanelles also offer a degree of protection against minor impacts to the head, particularly as infants begin to develop motor skills like holding their heads up, rolling over, and sitting. While appearing soft, these areas are covered by a tough fibrous layer, making them safe to touch gently. Healthcare professionals also use fontanelles to monitor a baby’s growth and health, as their appearance can indicate conditions such as dehydration (sunken fontanelle) or increased intracranial pressure (bulging fontanelle).
Do Adult Skulls Have Soft Spots?
Adult skulls do not possess the “soft spots” or fontanelles found in infants. As a person matures, the skull bones, initially separated by fontanelles and sutures, gradually fuse together through a process called ossification. This involves the flexible connective tissue of the sutures creating a rigid and protective structure around the brain.
The timing of this fusion varies among the different sutures. For instance, the metopic suture, which divides the frontal bone, typically fuses between 3 and 9 months of age, often being fully ossified by age 7. Other major sutures, such as the coronal, sagittal, and lambdoid sutures, generally begin to close later, with complete fusion largely occurring by early adulthood, around age 20 to 26 for some. However, research indicates that complete obliteration of these sutures rarely occurs, even in very old age, with most remaining somewhat patent.
While the adult skull is a hard, fused structure, certain areas of the head are not bony. For example, the nose and ears contain cartilage, which is a flexible connective tissue, but these are not considered part of the skull bone structure itself. The skull’s primary function in adulthood remains the protection of the brain.