VAN 121: Veterinary Gross Anatomy-II (2+2)

Neck - Ventral

NECK - VENTRAL ASPECT 

• Dissection: Make a longitudinal incision through the skin on the ventral median line commencing from the manubrium sterni to the transverse incision across the neck behind the angles of the jaw made in the dissection of submaxillary region. Reflect the flaps of the skin to the sides up to the ventral border of the brachiocephalicus. Great care should be taken in reflecting the skin to preserve the cutaneous nerves of this region.

• Cutaneous nerves of the neck: These are derived from the ventral primary branches pf the cervical spinal nerves from the second to the sixth. They appear to pierce through the brachiocephalicus at regular intervals and supply the ventral aspect of the neck.
• Sternocephalicus: It is a long muscle extending from the manubriumsterni to the skull. It consists of a superficial part, the sternomandibularis, which is very narrow, and a deep part, the sternomastoideus, which is broad.
• The muscle arises along with its fellow of the opposite side from the manubrium sterni and both the muscles completely cover the lower half of the neck. Above the middle of the neck, they diverge from each other. The sternomandibularis turns forwards below the angle of the jaw to be inserted to the masseter and the lateral face of angle of jaw. The sternomastoideus passes up deeply to be inserted to the mastoid process, wing of atlas and the basilar tubercles.
• Jugular furrow and external jugular vein: The jugular furrow is found on the ventrolateral aspect of the neck bounded by the sternomandibularis in front and brachiocephalicus behind. The external jugular vein lies in the furrow. It is formed by the union of the intermaxillary and superficial temporal veins on the deep face of the parotid salivary gland. At the angle of the jaw it receives the external maxillary vein and passes down the jugular furrow receiving small veins from the muscles. In front of the thoracic inlet it receives the cephalic vein and unites with the interal jugular vein to form the common jugular vein.
• Dissection: Cut the sternocephalicus across its middle and reflect the parts away; while reflecting the upper part towards the skull, secure the ventral branch of spinal accessory nerve passing through the brachiocephalicus below the wing of atlas and terminating in it. Secure the branches from the common carotid artery supplying the muscle. By careful search and dissection study the numerous structures mentioned below.
• Sterno-thyro-hyoideus: It arises with its fellow from the manubrium sterni and the muscles of either side cover the ventral face of the trachea. The sterno-cephalicus muscle in the lower half of the neck in turn covers them. Each muscle about the middle of the neck divides into two-bands, the thyroid and the hyoid bands which diverge from those of the opposite side in the upper third of the neck leaving ventral face of the trachea covered only by the fascia and the skin. The hyoid band is medial and narrow whereas the thyroid band to the lamina of the thyroid cartilage of the larynx.
• Omohyoideus: Its terminal part has been examined in the dissection of the submaxillary region. The muscle can now be followed back to its origin. It passes from the submaxillary region obliquely under the sternomastoideus and then under the anterior border of brachio-cephalicus to be attached to the second and third cervical vertebrae.

Note: The relation of omohyoideus to sternomastoideus varies in different specimens. While it may pass under or over the sternomastoideus in some specimens the fibres of the sternomastoideus offer difficulty in separating the muscles.

• Carotid sheath: This is on the dorsolateral aspect of the trachea. It is the deep cervical fascia, holds together the common carotid artery and vagosympathetic trunk. It is separated from the external jugular vein by the sternomastoideus muscle. In the undisturbed state, the sheath shows on its lateral face a small vein., the internal jugular vein and the tracheal lymph duct. Deeply in the sheath is the common carotid artery and on the dorsal aspect of the artery, the common cord of the vagus and sampathetic nerves. The carotid sheath on the left side is related to the oesophagus, which appears on the dorsolateral aspect of the trachea. The recurrent laryngeal nerve on the left side passes along the ventral border of the oesphagus. The recurrent laryngeal nerve on the right side passes along the ventral border of the carotid artery.
• Internal jugular vein: Is formed by the union of the occipital, thyroid and laryngeal veins. It passes down the neck superficially placed on the carotid sheath receiving the veins from the muscles, trachea and oesophagus. At the thoracic inlet, it joins the external jugular vein to form the common jugular vein.
• Tracheal duct: Is the efferent duct of the atlantal lymph gland. It passes superficially in the carotid sheath receiving the efferent from the cervical lymph glands placed along the carotid artery. The left duct joins the thoracic duct at the thoracic inlet. The right duct joins the anterior vena cava.

Note: The tracheal duct is not visible unless it is filled with lymph and so cannot be made out in many bodies.

• Common carotid artery: Arises from the bicarotid trunk at the thoracic inlet ventral to the trachea. Each artery passes up deeply in the jugular furrow along the dorsolateral aspect of the trachea detaching muscular branches and on the left side related to the oesophagus. It also gives of the thyrolaryngeal artery which divides into the thyroid and the laryngeal branches.
• The thyroid branch is the anterior thyroid artery, which divides into the thyroid artery entering the anterior extremity of the lobe of the thyroid. The laryngeal branch supplies some pharyngeal branches to the muscles of the pharynx and terminates by supplying the muscles of larynx. After giving the ventral parotid artery and posterior meningeal artery, the common carotid reaches the digastricus and terminates by dividing into the occipital external maxillary and external carotid arteries.
• Common cord of vagus and sympathetic: The vagus is the tenth cranial nerve which leaves the cranium by the foramen lacerum and joins the cervical sympathetic nerve to form the common cord of vagus and sympathetic which passes in the carotid sheath on the dorsal aspect of the common carotid artery and enters the thoracic inlet.
• Recurrent laryngeal nerve: It arises from the vagus in the thoracic cavity leaves the thoracic inlet and passes up the neck on the ventral aspect of the common carotid artery in the carotid sheath on the right side and supplies all the intrinsic muscles of the larynx except the cricothyroid. The nerve on the left side does not pass in the carotid sheath. It passes along the ventral border of the oesophagus in the fascia between it and the trachea and reaches the larynx.
• Oesophagus: It is a musculomembranous tube extending from the pharynx to the stomach. It arises from the auditus oesophagi of the pharynx above the cricoid cartilage and is placed on the trachea in the median line up to the fourth cervical vertebra, after which is passes to the left of the trachea related to the left carotid sheath dorsally and the recurrent laryngeal nerve ventrally and enters the thoracic inlet.
• Trachea: It is a large cartilaginous tube extending from the larynx on the median line of the neck into the thorax where it divides into two bronchi that enter the resepctive lung. It is flattened dorsally and is made up of "U" shaped runs of hyaline cartilage, the deficiency being made up of membrana trasnversa. At its origin from the auditus laryngis to the fourth cervical vertebra it is related to the oesophagus and dorsally to the longus colli muscle. Ventrally the sternothyrohyoideus muscles except cover it at its upper third. At its origin the lobes of the thyroid are placed on its lateral aspect with the isthmus passing across ventrally. The lobes of the thymus in the young subjects pass on its venterolateral aspects up to the thyroid covered by the sternothyrohyoideus muscles.
• Thymus: It is a lymphoid organ well developed in the foetus and the newborn. After birth it begins to degenerate. When present it has a body and two lobes made up of numerous lobules loosely connected by interlobular tissue. The body is situated in the thorax in front of the heart. The lobes arise from the body and extend through thoracic inlet on the ventrolateral aspects of the trachea reaching the lobes of the thyroid.
• Thyroid gland: It is a ductless gland placed on the first few rings of the trachea. It consists of two lobes placed on the lateral aspects of the trachea. The posterior ends of the lobes are connected by the isthmus, which passes across the trachea ventrally. The tissue of the thyroid is dark brown in colour. The lobes are oval and covered by the omohyoideus and sternocephalicus ventrally.
• Cervical lymph glands: They are small lymph glands placed along the carotid artery with numerous small haemolymph glands. They are classified as anterior, middle and posterior groups on the respective parts of the neck. Their afferents are from the muscles and other structures of the neck and their efferent ducts join the tracheal duct.
• Dissection: Cut across the trachea and oesophagus about the middle of the neck and reflect the cut ends of these organs in front and behind to examine the longus colli muscles covering the ventral aspects of the bodies of the cervical vertebrae.
• Longus colli: There are very long muscles covering the ventral aspects of the bodies of the cervical vertebrae. They extend from the tubercle of atlas, cover the bodies of the cervical vertebrae and extend under the bodies of the first six thoracic vertebrae.