Axial and Visceral Skeleton

Human skeleton development is very important from an evolutionary point of view; cephalization, also described as head formation, is one of the most important characteristics of vertebrates. The axial skeleton system commonly defines the human skull bone. The axial skeleton is defined as the skeleton system that consists of the human skull bones and bones of the trunk region. In this article, the axial and visceral skeleton of the human body is explained in detail. This article also focuses on the anatomy and importance of the cranial bones of the skull.

Axial and Visceral Skeleton 

The axial skeleton is the skeletal system of the vertebrates that consist of the bones of the head and trunk region. Axial skeleton presence in organisms is a characteristic feature showing that they belong to the vertebrate group. Thus the skeleton is also evolutionary very important. This skeleton consists of 80 bones combining all the bones of other structures. The axial and visceral skeleton consists of the following. 

1. Skull

2. Mandible

3. Hyoid

4. Vertebral column

5. Ribs

6. Sternum

Human Skull Bones

The human skull can be divided into three main regions, they are as follows, chondrocranium, splanchnocranium and, dermatocranium. The chondrocranium is the part of the human skull bone that encapsulates the sensory organ of the human body. The cranium is the region that comes under the chondrocranium part of the skeleton skull. The splanchnocranium is referred to as the visceral skeleton. They are known as the or visceral arches, these structures form the hyoid apparatus and the thyroid cartilage. The dermatocranium is the region of the dermal bones of the human skull bones that form the following braincase, jaws, and skeletal elements of the mouth (parts of teeth).

Cranial Bones of the Skull

The axial and visceral skeleton of the human skeleton system consists of the cranium of the skeleton skull. There are a total of eight cranial bones in the human skull. The cranium encloses the sensory organs of the human body. 

Cranial Bone Development

The cranium bones have two different origins; they are as follows, the cartilaginous, or substitution, and the membrane bones. The membrane bone is found in between the layers of the connective bones. The substitution bones are involved in the formation of the floor of the cranium while the membrane bones are involved in the formation of the roof and sides of the cranium.  There is variation in the bone of eight different types of cranial bone. They vary in the thickness of the bone. The carnival cavity of the human skull bones has irregularities. 

Bones of the Cranium

There are a total of eight bones that form the cranium of the brain skeleton. These bones are named as following

1.  the frontal bone,

2.  the sphenoid bone, 

3. the temporal bones,

4.  the parietal bones, and

5.  the occipital bone. 

There are two temporal bones and two parietal bones.

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Frontal Bone

 The frontal bone is present in the forehead region and from there it extends back to the coronal suture. There is a clear separation between the parietal and frontal bone; this separation is achieved by the presence of the arching line between these two bones. The frontal bone also forms a joint at the bridge of the nose, this joint involves two small bones one of them is the zygomatic bone, and the other is the bone of the nose bridge. The zygomatic bone is also responsible for the formation of the human face skeleton and its function. The frontal bone is also involved in the formation of the roof of the eye socket also known as the orbit. The function of this apparatus is to provide encasing to the sensory organ thus providing it with maximum protection. 

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Parietal Bone

The parietal bone is involved in the formation of the large portions of the sidewalls of the cranium. They are the most structurally distinctive bones, it can be said that they are exclusive to the cranium of the human skull and share very little similarity with the other bones. The parietal bones also perform the function of joining other bones of the cranium to each other; the parietal bone adjoins the frontal, the sphenoid, the temporal, and the occipital bones. Along the sagittal suture, the parietal bone forms a shallow depression called venous sinus. The venous sinus is the long cavity that is used for the venal blood circulation.

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Sphenoid Bone

 The sphenoid bone is the bone of the axial skeleton skull that has a butterfly shape. One of the important characteristics of the sphenoid bone is the presence of bony projections on its surface. The sphenoid bone consists of the following: body, paired greater wings and lesser wings, and two pterygoid processes. Another feature of this is the presence of a sphenoid cavity, which suggests that the body region of the bone is hollow. The cavity opens up in the nasal cavity. The projection or the irregularity on the body constitutes of the sella turcica and the chiasmatic groove. The greater wing region helps in the formation of three parts of the facial skeleton which are the floor of the middle cranial fossa, lateral wall of the skull and posterolateral wall of the orbit. The lesser wing is responsible for the formation of the lateral border of the optic canal.

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Temporal Bone

The temporal bone is involved in the formation of the lower lateral walls of the brain skeleton. Temporal bones are mainly responsible for the formation of the middle and inner portions of the ear. One of the characteristic features of the temporal bone is the rich supply of the cranial nerves. It is heavily associated with the cranial nerve supply to ensure the proper functioning of the muscles attached to the bone. The temporal bone consists of the five parts; they are as follows: squamous, tympanic and, petromastoid, zygomatic, and styloid. The zygomatic and styloid projects outwards. Squamous, tympanic and petromastoid parts constitute the majority of the bone.

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Occipital Bone 

The occipital bone is one of the most complex bones of the human face skeleton, it is primarily involved in the protection of the cerebellum and the occipital lobes of the cerebrum. It is associated with various muscles and ligaments. It has a characteristic trapezoidal shape. 

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The Hyoid Bone of the Axial Skeleton

The hyoid bone is a part of the axial skeleton system, they are serving the primary role of attachment. Hyoids are responsible for the attachment of the tongue. The hyoid body is made up of the larger horns, called the greater cornua, and a pair of smaller horns called the lesser cornua. One of the important features of the hyoid bone is the lack of any articulation with other bones. The hyoid is associated with the largest cartilage of the larynx. The hyoid can be described as a U shaped bone. The body forms the central part. They are also known as the lingual bone. There are three main ligaments attached to the hyoid bone: stylohyoid ligament, thyrohyoid membrane and, hyoepiglottic ligament.