The Horse's Spine

Following on from my blog about the bones of the skull last week, I wanted to find out more about a horse's spine.   The horse's spine is rigid during locomotion when you compare it to other mammals, for instance a cat or dog! The functions of the spine are to carry some of the body mass and also to transmit forward propulsion.

The spine contains the spinal cord - which is, basically, all the nerves coming from the brain.  There are 36 pairs of nerve roots which branch off from the spinal cord and exit the spine through the 'intervertebral foramen' - basically the space between 2 vertebrae!  These spinal nerve roots are the connecting structure between the Central Nervous System and the peripheral nerves (see my blog).

As I mentioned in my blog about the Horse's Skeleton the spine is made up of 7 cervical vertebrae, 18 thoracic, 6 lumbar, the sacrum and 15 - 22 coccygeal vertebrae.  All vertebrae have 3 basic components; the body, arch and spinous processes, these will be slightly different in each type of vertebra depending on the individual requirements.  The arch surrounds the vertebral canal and the processes supply a place for muscle attachment .  There are 3 types of processes; the spinous processes (point upwards), the transverse (which come out at right angles) and the articular processes which are smaller but also come out to the side.

 
The Cervical Vertebrae.

The first 2 of these have individual names as they are slightly different in structure to the rest. The Atlas is the first cervical vertebra and is designed to meet the occipital bone of the skull.  It has a larger hole (vertebral canal) because this area moves a great deal and so there is plenty of space for the spinal cord to prevent damage.  The joint between the skull and the Atlas allows the nodding movement of the head.

The second cervical vertebra is called the Axis.  This is designed to provide a place for the transverse ligament to attach and to join to the Atlas and the next cervical vertebra.  The large spinous process (divided into 2 ridges) allows for the attachment of the nuchal ligament - which is the strong and central neck ligament.  The nuchal ligament attaches to the nuchal crest of the occipital bone (as mentioned in last week's blog).  The joint between the Atlas and Axis allows some rotation of the head.

The other 5 cervical vertebrae are of a more standard design, with a body, and small spinous processes. The large transverse processes allow muscle attachment - particularly the serratus ventralis muscle.  The joints between these vertebrae allow lateral curvature and some arching of the neck.


The Thoracic Vertebrae.

These vertebrae provide surfaces for the heads of the ribs, in addition to protected passage for the spinal cord.  Each vertebra has a 'disc' in between made up of cartilage. As the horse ages this cartilage can become calcified too ie: turn to bone.  The size and direction of the spinous processes alters in each; the early thoracic vertebrae have large spinous processes which point backwards, the last 2 point forwards.   The spinous processes become smaller from the 5th onwards.  Thoracic vertebrae move very little in any direction.


The Lumbar Vertebrae

Although, there is usually 6, in some breeds eg: the Arab, there may only be 5 with an additional thoracic vertebra instead.  These vertebrae have long transverse processes.  The last few lumbar vertebrae are joined in such a way that there is very limited movement. 

The Sacrum

The 5 vertebrae of the sacrum are fused together.


The Coccygeal Vertebrae

The number can vary between 15 and 22.  With the exception of the first few these do not have vertebral canals or articular processes.   

It makes sense to include the ribs and sternum here too....


The Ribs

Usually there are 18 pairs of ribs, to correspond to the thoracic vertebrae.  These are relatively elastic and at the bottom end are made up of cartilage (costal cartilage).  The first 8 pairs articulate (join) with the sternum through this cartilage but for the last 10 the cartilage is bonded together to form the costal arch - and this links to the sternum!  The last rib is short and is often called a 'floating rib'.  The amount of curve in each rib increases from front to back of the horse.  The ribs move to allow an increase in capacity of the chest for respiration.    The first 9 ribs also provide attachment for the serratus ventralis muscle! 

 
The Sternum

This is held in position by the first 8 ribs.  It is a made up of a number of bony segments which are connected by cartilage.  The pectoral muscles are attached to the sternum and costal cartilage.  Forming the floor of the chest the sternum also gives a place for the attachment of some neck muscles. 

Next week's blog will be about the front leg!

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Until next time!

Jo