LAB 8 Neck

Almost all mammals have seven cervical vertebrae but in birds the number ranges from 11 to 25. Therefore, the shape of mammalian cervical vertebrae varies with the length of the neck but typically cervical vertebrae are longer than those in other spinal regions. Also, these vertebrae are more variable than those of other regions. The atlas (C1) and axis (C2) are the most variable of all. The atlas consists of two arches and is the only vertebra that lacks a body. The wings of the atlas make it the widest of all cervical vertebrae so that it is easily palpable caudal to the head. The axis is the longest of all vertebrae and has a well developed elongated spinous process. In contrast, other equine vertebrae have short or absent spinous processes but these are better developed in ruminants. The dens (tooth like) is a cranial projection of the body of the axis. The transverse processes are variable and complex. Caudal to the axis they have a ventral and lateral process. The ventral part of the transverse process is best developed on C5 and C6. These structures are landmarks for venipuncture in camelids. Near the vertebral arch the transverse processes have a transverse foramen that transmits the vertebral artery and nerve. These foramens are present on all cervical vertebrae except C7. The vertebral artery is the main artery to the neck because the common carotid artery bypasses the neck to supply the head.

The funicular nuchal ligament is continuous with the less elastic supraspinous ligament which can be considered to be the caudal attachment of the nuchal ligament. In the dog, the funicular nuchal ligament extends to the axis but in herbivores it reaches the skull. The laminar nuchal ligament extends to the axis and is absent in the dog. Like the cat, pigs lack the nuchal ligament. This may be due to their short necks. Both parts of the nuchal ligament are paired and are easily separated on the midline.

The nuchal fatty crest is unique to equine animals but has a tough fibrous structure similar to the fatty humps of old world camelids. The nuchal crest lies dorsal to the nuchal ligament and adjacent rhomboideus muscle in the horse but in cattle the rhomboideus muscle lies dorsal to the nuchal ligament.

In general, the muscles of the neck are either extrinsic forelimb muscles or they function to move the head and neck. However, all neck motion serves to move the head as well. Also, the neck muscles can be divided into epaxial muscles above the transverse processes or hypaxial muscles which lie ventral to the transverse processes. The omohyoideus muscle is absent in the dog, poorly developed in ruminants and best developed in the horse where it separates the external jugular vein and carotid artery in the cranial neck region. Traditionally the omohyoideus muscle has been claimed to be a barrier that protects the carotid artery from accidental puncture by a needle passing through the external jugular vein. However, it is unlikely that a sharp needle would be stopped by a muscle as thin as the omohyoideus.

The superficial cervical lymph node (formerly prescapular) is large and readily seen when enlarged due lymphoma in cattle or C. pyogenes abscess in goats and sheep. It lies deep to the omotransversarius m. in ruminants. Young pigs and ruminants have a well developed cervical thymus in the ventral neck in addition to the precardiac thoracic location. In guinea pigs the thymus is only found in the ventral neck.