Dicondylic and Monocondylic Skull – Definition, Functions

Dicondylic and Monocondylic skull refers to the two types of brain hemispheres that exist in human beings. They are categorized based on the number of cranium bones (cranial base) present in their skull. Both the skull types are similar in structure and thought process, but each turn out to be unique according to its anatomical location. This article attempts to explain the differences between these two skull types and their differences.

What is a Dicondylic Skull?

“Dicondylic ” refers to an individual having two bones, namely the “inter-parietal” and “Occipital” bones that form the cranium or the skull cavity for housing the brain. The Dicondylic Skull is also known as a Biomorphic Skull, meaning it has two forms. One of its forms resembles a Monosphenoid and the other form resembles that of a typical Ortho Sphenoid. The one which resembles a Monosphenoid is known as “Bodicullate”, while that resembling an Orthosphenoid is known as “Lophodont”.

The feature distinguishing both these types (i.e. the Dicondylic skull) is the presence of an inter-parietal bone that separates and envelopes the occipital bone, which houses the brain, from the parietal bone. This is termed a “suture” as per skeletal structure. The Occipital and Parietal bones are also joined together by a suture thus forming this biomorphic brain cavity. The Dicondylic Skull has two forms or variations; one being similar to that of a typical Monosphenoid (the Plastic Skull) while the other is similar to that of an Orthosphenoid (the Lophodont Skull).

What is a Monocondylic Skull?

“Monocondylic”  means having one skull bone or cranium, i.e. the “Parietal Bone”. The Monocondylic Skull is a “Mono Sphenoid” type of brain cavity that exists in some mammals, such as Whales and Manatees (Aquatic Mammals).

The brain cavity is formed by a single bone called the Parietal Bone that envelops and separates the Occipital bone from the Temporal bone. In Manatees, it is also known as a Plagioptychic Brain Cavity and exhibits unique properties when compared to other mammals (in this case humans). For instance, it forms a “unified chamber” that has no suture separating the brain from the skull cavity. Thus, this monosphenoid structure allows the brain to expand to much larger volumes compared to human beings.

Dicondylic and Monocondylic Skull: Similarities

Both have similar properties and characteristics. The most noticeable similarity between both is the presence of a homogenous cranial base without any sutures separating the bones of this skull. The other similarities are as follows:

1. As aforementioned, both Dicondylic and Monocondylic skulls have similar brain structures (and contents) in terms of volume and shape.
2. Both exhibit a similar form of the occipital bone referred to as “occipital condyle”.
3. The Dicondylic Cranium is further subdivided into four regions containing the brain; Prefrontal Cortex, Parietal Cortex, Temporal Cortex and Occipital Cortex.
4. The Dicondylic Occipital Bone has a distinct tubercle (PlatySphenoidLike) at its lateral condyle on the internal side (“Periosteal Surface ”). While in Monocondylic Skull, has no such distinguishing feature on its external surface but only on its external or the periosteal surface.
5. The Dicondylic Bipyramidal Cranium is further subdivided into three regions containing the brain; Frontal Cortex, Parietal Cortex, and Temporal Cortex.
6. The Dicondylic Occipital Bone has a distinct tubercle (PlatySphenoidLike) at its lateral condyle on the external surface (“Periosteal Surface”). While Monocondylic Skull, has no such distinguishing feature on its internal surface but only on its internal or the periosteal surface.

Dicondylic and Monocondylic Skull: Differences

1. The Dicondylic brain cavity is formed by a single bone while the Monocondylic one is formed from two bones.
2. The Dicondylic Brain Cavity forms a suture separating the parietal bone from the occipital bone. On the other hand, the Monocondylic Brain Cavity does not form any sutures.
3. The Dicondylic Cranium is further subdivided into four regions containing the brain; Prefrontal cortex, Parietal Cortex, Temporal Cortex and Occipital Cortex. The Monocondylic Cranium is subdivided into three regions; Frontal Cortex, Parietal Cortex and Temporal cortex.
4. The Dicondylic occipital bone has a distinct tubercle (Platysphenoidlike) at its lateral condyle on the internal side (“Periosteal Surface ”). While in Monocondylic Skull, it has no such distinguishing feature on its external or periosteal surface.
5. The Dicondylic Bipyramidal Cranium is further subdivided into three regions containing the brain; Frontal cortex, Parietal cortex, and Temporal Cortex. The Monocondylic Cranium is subdivided into two regions; Frontal cortex and Parietal cortex.
6. The Dicondylic occipital bone has a distinct tubercle (Platysphenoidlike) at its medial condyle (“Medial Surface”). While in Monocondylic Skull, it has no such distinguishing feature on its internal or periosteal surface.
7. The Dicondylic Occipital Bone has a distinct long trihedral eminence on the central or lower part of the internal surface. While in Monocondylic Skull, this feature is missing on both internal and external surfaces of the occipital bone.

Also See:

• Difference Between Simple and Complex Tissue
• Difference Between Plasmid DNA and Chromosomal DNA
• Difference Between Insect-pollinated and Wind-pollinated Flowers
• Difference Between Obligate Parasite and Facultative Parasite
• Difference Between Plant Tissue and Animal Tissue

Conclusion

The two types are diametrically opposite and hence constitute different variations of the same skull structure. The Dicondylic brain cavity is formed from an inter-parietal bone that is enveloped by the occipital bones. The Monocondylic Brain Cavity does not form any suture, but instead forms a monosphenoid or unified chamber for housing the brain contents without separating it from the cranial cavity. Thus, both have similar characteristics but at the same time have differences in their external and internal surfaces and other features of their cranial structures.