Part 119

TEETH ERUPTION

A. Deciduous: 1st incisor (Di 1) Birth or first week. 2nd „ (Di 2) 4–6 weeks. 3rd „ (Di 3) 6–9 months. Canine (Dc ) 1st premolar (Dp 2) Birth or first two weeks. 2nd „ (Dp 3) „ 3rd „ (Dp 4) „

B. Permanent:

1st incisor (I 1) 2½ years. 2nd „ (I 2) 3½ years. 3rd „ (I 3) 4½ years. Canine. (C ) 4–5 years. 1st premolar (or wolf tooth) (P 1) 5–6 months. 2nd „ (P 2) 2½ years. 3rd „ (P 3) 3 years. 4th „ (P 4) 4 years. 1st molar (M 1) 10–12 months. 2nd „ (M 2) 2 years. 3rd „ (M 3) 3½–4 years.

(The periods given for P 3 and 4 refer to the upper teeth; the lower ones may erupt about six months earlier.)

THE SALIVARY GLANDS

This term is usually restricted to the three pairs of large glands situated on the sides of the face and the adjacent part of the neck—the parotid, submaxillary, and sublingual. Their ducts open into the mouth.

The =parotid gland= (Glandula parotis) (Fig. 172)—so named from its proximity to the ear—is the largest of the salivary glands in the horse. It is situated chiefly in the space between the ramus of the mandible and the wing of the atlas. It is somewhat triangular in shape, the apex partially embracing the base of the external ear. Its length is about eight to ten inches (ca. 20 to 25 cm.), and its average thickness nearly an inch (ca. 2 cm.). Its average weight is about seven ounces (ca. 200 to 225 g.).

It presents for description two surfaces, two borders, a base, and an apex. The =external= (or superficial) =surface= is covered by the parotid fascia, the panniculus, and the parotido-auricularis muscle. It is crossed obliquely by the jugular vein, which is embedded in the gland tissue to a varying extent. It is also related to the great auricular vein, the cervical branch of the facial nerve, and branches of the second cervical nerve. The =internal= (or deep) =surface= is very uneven, and has numerous important relations. Some of these are: the guttural pouch, and the great cornu of the hyoid bone; the masseter, stylo-maxillaris, digastricus, and occipito-hyoideus muscles; the tendons of the mastoido-humeralis and sterno-cephalicus (which separate the parotid from the underlying submaxillary gland); the external carotid artery and some of its branches; the facial nerve; the pharyngeal lymph glands. The =anterior= (or facial) =border= is closely attached to the ramus of the mandible and the masseter muscle; it overlaps the latter to a varying extent. (In some cases there is a well-marked triangular facial process, which covers the temporo-mandibular joint, the facial nerve, and the transverse facial vessels.) The =posterior= (or cervical) =border= is somewhat concave, and is loosely attached to the underlying muscles. The =base= or =ventral border= is related to the external maxillary vein. The =apex= is attached to the base of the external ear, which it partially embraces.

The gland has a yellowish-gray color and is distinctly lobulated. It is inclosed in a capsule formed by the parotid fascia. The =parotid duct= (Ductus parotideus Stenonis) is formed at the lower part of the gland, near the facial edge, by the union of three or four radicles. It leaves the gland about an inch (ca. 2 to 3 cm.) above the external maxillary vein, crosses the tendon of the sterno-cephalicus, and gains the inner face of the pterygoideus internus. It then runs forward in the submaxillary space below the external maxillary vein and winds around the lower border of the mandible behind the vein, passes upward between the vein and the masseter muscle for about two inches (ca. 5 cm.), turns forward underneath the facial vessels, and perforates the cheek obliquely opposite the third upper cheek tooth. Before piercing the cheek it is somewhat dilated, but its termination is small, and is surrounded by a circular mucous fold (Papilla salivalis).

=Blood-supply.=—Branches of the carotid and maxillary arteries.

=Nerve-supply.=—Trigeminal, facial, and sympathetic nerves.

The =submaxillary gland= (Glandula submaxillaris) is much smaller than the parotid. It is long, narrow, and curved, the dorsal edge being concave. It extends from the fossa below the wing of the atlas to the body of the hyoid bone, so that it is covered partly by the parotid gland, partly by the lower jaw. Its length is about eight to ten inches (ca. 20 to 25 cm.), its width an inch to an inch and a half (ca. 2.5 to 3 cm.), and its thickness about half an inch (ca. 1 cm.). It weighs about one and a half to two ounces (ca. 45 to 60 g.). It is often divisible into two parts.

It presents for description two surfaces, two borders, and two extremities. The =external surface= is covered by the parotid gland, the stylo-maxillaris, digastricus, and pterygoideus internus muscles. The tendon of the sterno-cephalicus crosses this surface, and is a useful guide in separating the parotid gland from it. The =internal surface= is related chiefly to the flexor muscles of the head; the guttural pouch; the larynx; the division of the carotid artery; and the tenth, eleventh, and sympathetic nerves. The =superior border= is concave and thin. It is related to the guttural pouch and the duct of the gland. The =inferior border= is convex and thicker. It is related to the thyroid gland and the external maxillary vein. The =posterior extremity= is loosely attached in the fossa atlantis. The =anterior extremity= lies at the side of the root of the tongue, and is crossed externally by the external maxillary artery.

The =submaxillary duct= (Ductus submaxillaris Whartoni) is formed by the union of small radicles which emerge along the concave edge. It runs forward along this border, and, after leaving the anterior extremity, crosses the intermediate tendon of the digastricus, passes between the hyo-glossus and mylo-hyoideus, and gains the inner surface of the sublingual gland. Its terminal part lies on the body of the mandible, under the mucous membrane, which it pierces opposite the canine tooth. The orifice is at the end of a flattened papilla (Caruncula sublingualis).

=Blood-supply.=—Occipital, external carotid, and external maxillary arteries.

=Nerve-supply.=—Chorda tympani and sympathetic nerves.

The =sublingual gland= (Glandula sublingualis) is situated beneath the mucous membrane of the mouth, between the body of the tongue and the ramus of the mandible. It extends from the symphysis to the third or fourth lower cheek tooth. Its length is about five or six inches (ca. 12 to 15 cm.) and its weight about half an ounce (ca. 15 to 16 g.).

It is flattened laterally, and has a thin =upper border= which causes an elevation of the mucous membrane of the floor of the mouth, termed the =sublingual crest= (Plica sublingualis). The =external surface= is related to the mylo-hyoideus muscle, and the =internal surface= to the genio-glossus and stylo-glossus, the submaxillary duct, and branches of the lingual nerve. The =inferior border= is related to the genio-hyoid muscle.

The =sublingual ducts= (Ductus sublinguales minores), about thirty in number, are small, short, and twisted; they open on small papillæ on the sublingual crest.

=Blood-supply.=—Sublingual artery.

=Nerve-supply.=—Trigeminal and sympathetic nerves.

THE PHARYNX

The pharynx is a musculo-membranous sac which belongs to the digestive and respiratory tracts in common. It is funnel-shaped, the base joining the mouth and nasal cavity, while the apex is continued by the œsophagus. Its long axis is directed obliquely downward and backward, and has a length of about six inches (ca. 15 cm.).

The pharynx is attached by its muscles to the palate, pterygoid, and hyoid bones, and to the cricoid and thyroid cartilages of the larynx.

Its principal =relations= are: dorsally, the base of the cranium and the guttural pouches; ventrally, the larynx; laterally, the internal pterygoid muscle, the great cornu of the hyoid bone, the internal and external maxillary arteries, the glosso-pharyngeal, superior laryngeal, and hypoglossal nerves, the submaxillary salivary gland, and the pharyngeal lymph glands.

It presents =seven openings=. The =posterior nares= (Choanæ) communicate dorsally with the nasal chambers. The pharyngeal orifices (Ostia pharyngea) of the two =Eustachian tubes= are situated on the lateral wall behind the nares and a little below the level of the inferior nasal meatus. They are slit-like openings, slightly oblique downward and backward, and are a little more than an inch (ca. 3 cm.) in length. They are bounded internally by a valvular flap formed by the expanded extremity of the cartilaginous Eustachian tube. The =isthmus faucium= is situated below and in front. It is closed by the soft palate except during swallowing. The =laryngeal orifice= (Aditus laryngis) occupies the greater part of the ventral wall or floor of the pharynx. Behind this is the =œsophageal opening=.

The wall of the pharynx comprises from without inward: the muscles, the pharyngeal aponeurosis, and the mucous membrane.