Carnivore

Thyroid gland and Parathyroid glands

The thyroid gland is dark-colored and usually consists of two separate lobes lying lateral to the first five tracheal rings. Occasionally, a connecting isthmus is present.

There are two parathyroid glands associated with each thyroid lobe. They are small, light-colored, spherical bodies. The external parathyroid most commonly lies in the fascia at the cranial pole of the thyroid lobe. It may be entirely separate from the thyroid tissue or embedded in the cranial pole of the thyroid external to its capsule. The internal parathyroid lies deep to the thyroid capsule on the medial aspect of the lobe. Occasionally, it is embedded in the parenchyma of the thyroid and is difficult to locate. The location of these glands is subject to variation.

Common carotid artery and its branches; Medial Retropharyngeal Lymph Node

WHILE FOLLOWING THE COMMON CAROTID ARTERY AND ITS BRANCHES, RECALL THAT  CRANIAL NERVES IX-XII and THEIR BRANCHES AND ASSOCIATED STRUCTURES, ARE PROMINENT STRUCTURES IN THIS REGION.  THEY WERE DESCRIBED IN DETAIL IN THE CRANIAL NERVE SECTION OF THE NERVOUS SYSTEM UNIT.  TAKE THIS OPPORTUNITY TO REVIEW THESE NERVES AS YOU NOW LEARN THE ARTERIES LOCATED IN THIS REGION.

1.   FYI:  The caudal thyroid artery has a variable origin from the major arterial branches in the thoracic inlet. It passes cranially on the trachea supplying the trachea, esophagus, and thyroid gland.   It  need not be dissected.
   
2. The cranial thyroid artery arises from the ventral surface of the common carotid at the level of the larynx. It supplies the thyroid and parathyroid glands, the pharyngeal muscles, the laryngeal muscles and mucosa, the cervical parts of the trachea and esophagus, and the adjacent portions of the sternocephalicus and the mastoid part of the cleidocephalicus.

The large medial retropharyngeal lymph node is dorsal to the common carotid artery at the larynx and ventral to the wing of the atlas. Afferent vessels arise from the tongue, the nasal cavity, the pharynx, the salivary glands, the external ear, the larynx, and the esophagus. The tracheal trunk of each side arises from this lymph node.

3.  The internal carotid artery is closely associated with the occipital artery, the first branch of the external carotid. FYI:  A bulbous enlargement at the origin of the internal carotid artery is the carotid sinus, a baroreceptor. (The carotid body, a chemoreceptor, lies at the bifurcation of the carotid arteries.) Beyond this the internal carotid artery ascends across the lateral surface of the pharynx medial to the occipital artery. No branches leave the internal carotid in its extracranial course. It enters the carotid canal deep in the tympano-occipital fissure. Ultimately, it branches to form the cerebral arterial circle that supplies the brain. Its branches to the brain will be observed later in the lab.

4. The occipital artery leaves the external carotid adjacent to the internal carotid and passes dorsally to supply the muscles on the caudal aspect of the skull and the meninges.

5.  FYI:  The cranial laryngeal artery is a ventral branch that supplies the adjacent sternomastoideus and pharyngeal muscles. It enters the larynx between the thyrohyoid bone and the thyroid cartilage to supply the mucosa and laryngeal muscles.

6.  The lingual artery leaves the ventral surface of the external carotid and passes rostrally to supply the tonsil and tongue. On its course to the tongue it is accompanied by the hypoglossal nerve.

7.  The facial artery leaves the external carotid beyond the lingual, medial to the digastricus. FYI:  A branch, the sublingual artery, continues medial to the digastric muscle and is accompanied by the mylohyoid nerve. It runs rostrally into the tongue. The facial artery courses rostrolaterally between the digastric and masseter muscles to reach the cheek lateral to the mandible, where it supplies the lips and nose.  Be sure to identify it on the superficially dissected head.

I don’t know how to get number eight to ident like the others.  I cut and pasted it from before the blue box to after.  

8.  The caudal auricular artery usually arises from the external carotid at the base of the ear and courses dorsally under the caudal auricular muscles.  Lateral, intermediate, and medial auricular branches course distally on the convex caudal surface of the ear. Occasionally, this artery arises closer to the origin of the external carotid.

At the caudal border of the mandible, rostroventral to the annular cartilage of the ear, the external carotid artery terminates by dividing into the superficial temporal and maxillary arteries.

9.  The superficial temporal artery arises rostral to the base of the auricular cartilage at the caudodorsal border of the mandible and courses dorsally. It supplies the parotid gland, the masseter and temporal muscles, the rostral auricular muscles, and the eyelids.

Maxillary Artery and Branches

The maxillary artery is the larger terminal branch of the external carotid artery. It is deeply placed and is closely associated with a number of cranial nerves. From its origin with the superficial temporal, it passes rostromedially ventral to the temporomandibular joint medial to the retroarticular process in its course to the alar canal.  It enters the the alar canal via the caudal alar foramen and exits via the cranial alar foramen along with the maxillary nerve (branch of CNV), which exits the skull into the alar canal via the round foramen.

NOTE that the cat does not possess an alar canal!

The first three arise before the maxillary artery enters the alar canal.

1. 1. The inferior alveolar artery enters the mandibular foramen with the inferior alveolar nerve and courses through the mandibular canal. It supplies branches to the roots of the teeth in the lower jaw. Mental branches supply the skin.

2.  FYI:  The caudal deep temporal artery arises near the inferior alveolar artery and enters the temporal muscle. Only the origin of this artery may be seen.

3.  FYI: The middle meningeal artery passes through the oval foramen and courses dorsally in a groove on the inside of the calvaria. 

FYI: The external ophthalmic artery arises from the maxillary on its emergence from the alar canal and penetrates the apex of the periorbita adjacent to the orbital fissure. The external ophthalmic artery gives rise to the vessels that supply the structures within the periorbita.  You also may observe the buccal artery as it passes cranioventrally adjacent to the buccal nerve.

FYI:  In both the dog and cat, you may observe the minor palatine artery as it branches from the maxillary.  The minor palatine artery passes ventrally, caudal to the hard palate, and is distributed to the adjacent soft and hard palates.  Just caudal to the maxillary foramen, the maxillary artery bifurcates into the descending palatine and infraorbital arteries, the former of which becomes the main channel artery to the rostral head.

The infraorbital artery supplies the malar artery to the eyelids and dental branches to the caudal cheek teeth. It then enters the maxillary foramen and passes through the infraorbital canal. Within the canal, dental branches arise. These supply the premolars, the canine teeth, and the incisor teeth. The infraorbital artery emerges from the infraorbital foramen and terminates as the lateral and rostral dorsal nasal arteries, which supply the nose and the superior lip.

Now, move back to where the maxillary artery split into the descending palatine and the infraorbital artery.   

FYI: The sphenopalatine artery branches from the descending palatine and passes through the sphenopalatine foramen to the interior of the nasal cavity.   It need not be dissected.

The major palatine artery, the continuation of the descending palatine beyond the sphenopalatine, represents the main channel artery to the hard palate.  It enters the caudal palatine foramen and passes through the major palatine canal to supply the hard palate.

Vasculature of the Brain

The arteries to the cerebrum and cerebellum are branches from the vessels on the ventral surface of the brain. These major arteries are not accompanied by veins. The basilar artery is formed by the terminal branches of the vertebral arteries, which enter the floor of the vertebral canal through the lateral vertebral foramina of the atlas.  It is continuous caudally with the ventral spinal artery of the spinal cord. The basilar artery courses along the midline of the ventral surface of the medulla oblongata and pons and then divides into two branches that form the caudal portion of the arterial circle of the brain (eponym: “circle of Willis”).

The internal carotid arteries are the other main source of blood to the arterial circle of the brain (not in the cat or the ruminant). Remember that in the cat, the other main source of blood to the arterial circle is the maxillary rete mirabile.  Each internal carotid artery divides into a middle cerebral, rostral cerebral, and caudal communicating artery. The small caudal communicating arteries from each internal carotid course caudally and join the terminal branches of the basilar artery. Rostrally, the two rostral cerebral arteries anastomose, completing the arterial circle on the ventral surface of the brain.

The cerebral arterial circle surrounds the pituitary gland, which receives small branches from the circle as well as directly from the internal carotid artery.

Venous sinuses are large veins in the brain that drain blood from the brain to the heart. They are located between the layers of the dura mater, the brain’s outermost covering. On one half of the head, a fold of dura will be found extending ventrally from the midline in the longitudinal cerebral fissure between the two cerebral hemispheres. This is the falx cerebri, which contains the dorsal sagittal sinus dorsally. The paired cavernous sinus lie on the respective sides of the floor of the middle cranial fossa. The two cavernous sinuses are connected medially by means of the large but short rostral and caudal intercavernous sinuses.

Include cut cadaver head pointing to dorsal sagittal sinus.  Include photo of cavernous sinus model. 

Superficial Veins of the Carnivore Head and Mandibular Lymph Nodes

If students are using new heads again, consider moving these sections up to the beginning of the lab as description for the “superficial head dissection.” 

The external jugular vein is formed by the confluence of the linguofacial and maxillary veins caudal to the mandibular salivary gland, which lies between these two veins.

The maxillary vein drains the ear, orbit, palate, nasal cavity, cheek, and mandible as well as the cranial cavity.

The lingual vein is the first large tributary that enters the linguofacial vein ventrally. Its branches drain blood from the tongue, the larynx, and part of the pharynx. These branches will not be dissected.

The facial vein is the other tributary of the linguofacial. The branches that form the facial vein lie on the dorsal surface of the muzzle. FYI:  One of these, the dorsal nasal, runs caudally from the nares, whereas another, the angularis oculi, passes rostrally from the medial aspect of the orbit, where it is continuous with the ophthalmic plexus within the periorbita. Blood may drain from the face in either direction through the angularis oculi. There is also a large communication between the ophthalmic plexus and the facial vein, via the deep facial vein, which is ventral to the zygomatic bone.

Lymph Nodes of the Carnivore Head

Recall that the large medial retropharyngeal lymph node is dorsal to the common carotid artery at the larynx and ventral to the wing of the atlas. It should have been located during the study of the common carotid artery and its branches.  Afferent vessels arise from the tongue, the nasal cavity, the pharynx, the salivary glands, the external ear, the larynx, and the esophagus. The tracheal trunk of each side arises from this lymph node.

The two or three small mandibular lymph nodes are located along the linguofacial vein just rostroventral to the mandibular salivary gland. The salivary gland will be studied in the alimentary unit.  The afferent lymph vessels to the mandibular lymph nodes come from all parts of the head not drained by the afferent lymph vessels of the parotid lymph node. The efferent lymph vessels of the mandibular lymph nodes go primarily to the ipsilateral medial retropharyngeal lymph node.

The parotid salivary gland lies between the mandibular gland and the ear and will be studied with the alimentary system. It is closely applied to the base of the auricular cartilage of the ear.  A small parotid lymph node may be found along the rostral border of this gland.

The afferent lymph vessels to the parotid lymph node come from the cutaneous area of the caudal half of the dorsum of the muzzle and the side of the cranium, including the eyelids and associated glands, the external ear, the temporomandibular joint, and the parotid salivary gland. The parotid lymph center also drains the temporal, masseter, and zygomatic muscles; the muscles of the ear; the lacrimal apparatus; the nasal, frontal, parietal, zygomatic, and temporal bones; and the mandible. Its two or three efferent lymph vessels run between the digastric muscle and the parotid salivary gland to the large medial retropharyngeal lymph node. This flow can also drain into the lateral retropharyngeal lymph node, if the latter is present.

The lateral retropharyngeal lymph nodes are rarely missing in the cat.  They are located just caudal to the parotid salivary gland, at the ventral border of the wing of the atlas, against the dorsal border of the mandibular gland, under the base of the ear.

FYI: Lateral retropharyngeal lymph nodes are present in approximately 30% of dogs. This lymph node is less than 10 mm in diameter and lies at the dorsal border of the horizontal part of the cartilaginous external acoustic meatus. It is completely or partially covered by the caudal part of the parotid salivary gland. The afferent lymph vessels of the lateral retropharyngeal nodes come from the structures lying adjacent to them. Their efferent vessels drain into the medial retropharyngeal lymph node.

Vasculature and Lymph Nodes of the Ungulate Head

Superficial Vasculature of the Equine and Ruminant Head

NOTE:  For this section, we will be focusing on the equine and the small ruminant as the specimens which have latex in their vessels.  You need not identify the vasculature in the bovine or porcine.  However, you are responsible for the lymphatic, thyroid and parathyroid anatomy on the all of the ungulate species.

When available, observe this section with one equine specimen and one goat specimen on which a superficial dissection has been performed.

Clinical Applications:  Superficial Arteries of the Ruminant Head

• The cornual a. branches can bleed extensively during dehorning of a mature animal.  Do you recall the innervation to the horn?
• The pulse may be palpated on the facial (in bovine) or transverse facial a.

Superficial Veins of the Ungulate Species

Superficial Lymph Nodes of the Equine, Ruminant, and Porcine Head

Deep Arteries of the Ungulate Head

Equine

Identify the cranial thyroid a. (look for it heading straight toward the thyroid gland).

Continue cranially along the common carotid artery until the internal carotid a. branches from it.  Once the internal carotid has branched from the common carotid, the latter changes name to the external carotid artery, which is the main channel artery.

The first branch of the external carotid artery is the occipital a. Note that the occipital artery is aptly named and will head toward the occipital region of the head. Although it branches from the main channel cranial to the internal carotid artery, it runs more caudally.

To visualize  the following structures, disruption of the wall of the guttural pouch is necessary as viewed from the medial side.  Remember the guttural pouches, which were also reflected to observe cranial nerves, are large, auditory-tube diverticula.  They will be covered in depth during the respiratory unit.  Removal of the longus capitis m. insertion should have been completed during the cranial nerve dissection. From here, you can opt to flip to the lateral side to keep following the external carotid a. , or stay on the medial side until you can no longer move cranially along the external carotid.

You are responsible for identifying the following branches of the external carotid artery in the equine:

Occipital artery (heading toward the occipital region)

Linguofacial Trunk

Superficial Temporal Artery (will have been transected so the temporalis muscle could be removed).  Find the transverse facial artery, which is the first branch of the superficial temporal artery.

The continuation of the external carotid artery beyond the superficial temporal artery is the maxillary artery, which becomes the main channel artery at this location.

Branching from the maxillary artery, observe the inferior alveolar artery, running with the nerve of the same name, on its way to the mandibular foramen, through which it enters the mandibular canal.

The descending palatine artery branches from the maxillary artery more distally and becomes the main channel artery.  Its continuation beyond the sphenopalatine artery is the major palatine artery.  You need not observe these arteries as they branch from the maxillary artery, but you should observe the major palatine artery in the mouth, where the hard palate has been elevated to reveal it at that location.

Ruminant

While you are not required to identify all of the deep arteries in the ruminant, there are some significant species differences for which you will be responsible.

In the ruminant, as in the cat, the internal carotid a. degenerates to a fibrous cord in the neonate and its remnant may not be apparent at all. The occipital a. should be readily identifiable. The main blood supply to the brain is via the epidural rete mirabile.

Because the small ruminant does not possess a facial artery, the analogous artery branching from the external carotid artery is called the lingual artery.  Note that the bovine species do possess a facial artery, and thus also possess a linguofacial trunk.

Deep veins of the Equine Head

At the ventral mandible on its medial aspect dissect and identify the facial a. and v., passing with the parotid sg. duct and then crossing over the ventral cortex of the mandible and traveling dorsolaterally along the rostral margin of the masseter m. Zero in on the facial v. path along the rostral masseter.

The masseter muscle will have been resected in order to visualize the large veins and their sinuses that emerge from under this muscle to join the facial v.  From dorsal to ventral they are 1. the transverse facial vein (look just ventral and adjacent to the facial crest), 2. the deep facial vein, and 3.  the buccal vein.

Deep Lymph Nodes of the Ungulate Head

Ungulate Thyroid Gland and Parathyroid Glands

Thyroid Gland

There is only one thyroid gland per animal, not right and left thyroids, as is frequently implied.  In most mammals, the gland is divided into right and left lobes, connected ventrally by a variably developed isthmus, and species variation in shape exists  In the horse, each lobe is distinctly ovoid and may be palpable in the live animal; certainly it is palpable when enlarged due to thyroid adenoma development. In goats the thyroid is somewhat flat and elongate.

Parathyroid Glands

There are typically two pairs of parathyroid glands, the external parathyroids and the internal parathyroids. The internal parathyroids are typically very closely associated with or embedded in the thyroid gland. The external parathyroids may be in the vicinity of the thyroid gland (e.g. medial to the common carotid bifurcation in the ox) or some distance away e.g. in the horse, at the other end of the neck close to the caudal deep cervical lnn.!! Do not be too concerned with identification of these glands however do give a shout if you happen across any!