Lab Overview and Instructions:

The axial skeleton consists of all the bones that are not part of the limbs (the appendicular skeleton).  We can divide it into the skull, the hyoid apparatus, the vertebral column (with its regional subdivisions), the bones of the thoracic cavity wall (ribs, sternebrae), and also include the os penis.  The bones of the pelvis, although directly associated with back and axial skeleton anatomy, are considered part of the pelvic limb (appendicular skeleton) and will be covered in that unit.

Skull introduction

The skull is a conglomeration of approximately 29 bones, depending on how a ‘separate’ bone is defined.  There are many, many features of the skull and for this section and unit we are focused on an understanding of the bones that combine to form the skull.  Most holes in the skull (foramina, fissures, canals) will be considered in the Nervous System and Cardiovascular System, in association with the nerves and vessels that pass through those features.  Similarly, the air-filled sinuses (paranasal sinuses) found within certain bones will be considered in detail in the Respiratory System.

So, a place to start is to pick up a skull and to use the images below to identify the bones.  Any species can be selected as a starting point and once names and locations of bones are familiar this knowledge can be applied to the skulls of other species.  There can be slight variation between anatomy textbooks in the labeling of bones of the skull, in particular the bones of the sphenoid complex on the ventral aspect of the skull – use the terms list to guide your learning.

Carnivore skull

Horse Skull

Ruminant Skull

Pig skull

Comparative skulls

Hyoid apparatus

This is a construct of bones that attach the root of the tongue and larynx to the base of the skull.  ‘Hyoid’ is from the root words ‘hyo’-[Gr] related to U, the letter Upsilon, and ‘-eidos’ [Gr] shape or form. Therefore the hyoid is the U-shaped set of bones. The hyoid apparatus is flexible, moving with swallowing, for example. This structure will be revisited in future body systems (e.g. the Respiratory System) because of its close relationship to the larynx and the equine guttural pouch.

Vertebral column introduction

Individual vertebra, joined end to end, form the vertebral column.  A vertebra consists of a body, an arch, transverse processes, a (dorsal) spinous process, and articular processes.  The vertebral arch is formed by pedicles (the lateral walls) and laminae (the dorsal roof of the arch from which the spinous process projects from). The large central opening through each vertebra, bordered by the body and arch, is the vertebral foramen. The continuous bony tunnel of the vertebral column, created by adjacent vertebral foramina, is called the vertebral canal.  The spinal cord is located within this canal.  Laterally, between consecutive vertebrae, there is a space, the intervertebral foramen, for the passageway of spinal nerves exiting the vertebral canal to pass into the peripheral tissues.  The vertebral column has cervical, thoracic, lumbar, sacral and caudal subdivisions. Vertebrae within these subdivisons generally share common features.  The number of vertebrae in a subdivision is consistent for the cervical region (7), and then there is variation between species for the remaining divisions.  Formulas for the vertebral column are provided in the terms list and use the terms list to guide the learning of the features of vertebrae.

Carnivore vertebral column

Ungulate comparative vertebral column

Vertebral column features

Besides muscles, the axial skeleton is supported and connected by many ligaments and specialized and standard joint structures. We will focus on identifying a few major features.  The atlanto-occipital and atlantoaxial joints are synovial joints. These joints are continuous with each other through the articulation of the dens of the axis with the ‘floor’ of the atlas vertebral foramen.  The dens of the axis is held in place by the transverse ligament of the atlas (present in the carnivore and pig) and a few other ligaments. The atlanto-occipital joint is a hinge joint, moving dorsally and ventrally, i.e. the “yes” joint.  The atlantoaxial joint is a rotating joint, moving head side to side, i.e. the “no” joint.  A dorsal atlanto-occipital ligament (a membrane anatomically, but will keep it straightforward and call it a ligament) and dorsal atlantoaxial ligament provide fibrous connections between the associated bones.  When accessing the vertebral canal at these joint spaces, the dorsal ligament is penetrated.

At the C2-C3 articulation, and moving caudally, adjacent vertebrae articulate via synovial joints at the articular processes and via a fibrous joint between vertebral bodies.  The fibrous joint consists of the intervertebral disk.  The intervertebral disk is constructed of a central gelatinous core, the nucleus pulposus, and an outer ring of collagenous fibers, the anulus fibrosus.  Recall that the developed sacrum is typically a fused unit of multiple sacral vertebra and therefore synovial joints and intervertebral disks are not present between the sacral segments.

Continuing caudally from the nuchal ligament of the neck region, the supraspinous ligament connects the dorsal aspects of all vertebral spines from T1 vertebra through to the sacral vertebrae. The ligament passes from one spinous process tip to another.  A reminder, the vertebral arch consists of the vertebral laminae and pedicles, forming the roof and lateral walls of the vertebral foramen.  A ligament connects one vertebral arch to the next along the dorsal margin of the adjacent arches.  This is the yellow ligament (ligamentum flava) or sometimes, the interarcuate ligament (‘between arches’).  Lastly, in the thoracic region, the intercapital ligament extends between and left and right paired rib heads, holding the rib head to its joint.  The intercapital ligament crosses dorsally over the intervertebral disk and therefore possibly helps to keep the disk in its location (i.e. disk protrusion in the thoracic vertebral column is less common where the intercapital ligament is present, for the dog this is ribs 2 to 10).