Part 117

FIG. 243.—MUSCLES OF TONGUE, PHARYNX, LARYNX, ETC., DEEP DISSECTION.

T. p., Tensor palati; L. p., levator palati, concealed part indicated by dotted line; Pt. p., pterygo-pharyngeus; P. p., palato-pharyngeus; St. p., stylo-pharyngeus; C. a., crico-arytenoideus post.; C. th., crico-thyroideus; Th. c., thyroid cartilage (wing); Th. h., thyro-hyoideus; K. h., kerato-hyoideus; Th. c., thyroid cornu; S. c., small cornu; L. p., lingual process; F. p., foliate papilla; Fu. p., fungiform papillæ. Part of great cornu is removed and indicated by dotted line. ]

2. =Hyo-glossus.=—This is a wide, flat muscle, somewhat thicker than the preceding. It lies on the lateral part of the root and body of the tongue, partly under cover of the preceding muscle. Its deep face is related to the genio-glossus. It arises from the lateral aspect of the hyoid bone, from the lingual process to the oral extremity of the great cornu, and from the thyroid cornu. The fibers pass obliquely forward and upward, and for the most part turn toward the median plane of the dorsum of the tongue. Its action is to retract and depress the tongue.

(It is usually possible to recognize in this muscle three portions, which would correspond to the baseo-, kerato-, and chondro-glossus of human anatomy.)

3. =Genio-glossus.=—This is a fan-shaped muscle, which lies parallel to the median plane of the tongue. It is separated from the muscle of the opposite side by a quantity of fat and connective tissue. It arises from the inner surface of the ramus of the mandible near the symphysis. From the tendon the fibers pass in a radiating manner, some toward the tip, others toward the dorsum, and others toward the root of the tongue; some fibers pass from the posterior end of the tendon to the body and small cornu of the hyoid bone. The muscle as a whole is a depressor of the tongue, and especially of its middle portion; when both muscles act, a median groove is formed on the dorsum. The posterior fibers protrude the tongue, the middle fibers depress the tongue, and the anterior fibers retract the tip of the tongue.

=Vessels and Nerves.=—The arteries of the tongue are the lingual and sublingual branches of the external maxillary artery. The sensory nerves are the lingual and glosso-pharyngeal, and the muscles are innervated by the hypoglossal nerve.

THE TEETH

The teeth are hard white or yellowish-white structures, implanted in the alveoli of the bones of the jaws—premaxilla, maxilla, and mandible. Morphologically they are large calcified papillæ. Functionally they are organs of prehension and mastication, and may serve as weapons of offense and defense. They are classified according to form and position as follows:

1. =The incisor teeth= (Dentes incisivi) are situated in front and are implanted in the premaxilla and mandible.

2. =The canine teeth= (Dentes canini) are situated a little further back, at or near the premaxillary suture in the upper jaw; in the lower jaw they are nearer the incisors.

3. =The cheek teeth= (Dentes premolares et molares), the remaining teeth, occupy the sides of the dental arch. The space between the incisors and cheek teeth is termed the interdental or interalveolar space. It is customary to divide the cheek teeth into an anterior series, termed =premolars=, which appear as temporary, deciduous, or milk teeth, and are replaced by permanent successors, and a posterior series, the =molars=, which appear only as permanent teeth without deciduous predecessors.

As the teeth of the two sides of the jaws are alike in number and character (in normal cases), the complete dentition may be briefly indicated by a =dental formula= such as the following:

( 2 1 2 3 ) 2 ( I - C - P - M - ) = 32. ( 2 1 2 3 )

In this formula the letters indicate the kinds of teeth, and the figures above and below the lines give the number of teeth of one side in the upper and lower jaw respectively in man.

The individual teeth of each group are designated numerically, the starting-point being the middle line; thus the incisor on either side of the middle line is the first incisor, and may be conveniently indicated by the notation I^1. The temporary or deciduous teeth may be designated in a similar manner, prefixing D (for deciduous) to the letter indicating the kind of tooth. In addition to the above systematic method of notation other terms have received the sanction of popular usage. Thus the first incisors are commonly called “pinchers,” or “nippers”; the second, intermediate; and the third, corner teeth. The canines, when highly developed, may be termed tusks or fangs. The vestigial and inconstant first premolar of the horse is popularly termed the “wolf tooth.”

Each tooth presents for description a portion coated with enamel, termed the =crown= (Corona dentis), and a portion covered with cement, termed the =root= (Radix dentis). The line of union of these parts is the neck (Collum dentis). In many teeth the neck is distinct and is embraced by the gum, e. g., the teeth of the dog and the temporary incisors of the horse. In other teeth no constriction is seen, as in the permanent incisors of the horse. Between these extremes may be noted the molars of the horse, in which the neck is seen only in advanced age.

The =surface= of a tooth directed toward the lips is termed =labial=; toward the cheek, =buccal=; and toward the tongue, =lingual= (Facies labialis, buccalis, lingualis). The surface opposed to a neighboring tooth of the same dental arch is termed the =contact surface= (Facies contactus). The grinding or =masticatory= or “table” surface (Facies masticatoria) is that which comes in contact with a tooth or teeth of the opposite jaw.

=Structure.=—Teeth are composed of four tissues, which are considered here from within outward. The =pulp= (Pulpa dentis) is a soft gelatinous tissue, which occupies a space in the central part of the tooth termed the =pulp cavity= (Cavum dentis). The pulp is well supplied with blood-vessels and nerves. It occupies a relatively large space in young growing teeth, but later the dentine deposited on its surface gradually encroaches on it until, in advanced age, the cavity is obliterated or much reduced. The =dentine= (Substantia eburnea) forms the bulk of most teeth, covering the surface of the pulp. It is very hard, and is yellowish-white in color. The =enamel= (Substantia adamantina) constitutes a layer of varying thickness covering the dentine of the crown of the tooth. It is easily distinguished by its clear bluish-white appearance and its extreme density. The =cement= (Substantia ossea) is always the outermost tooth substance. In simple teeth it forms usually a thin layer on the surface of the dentine of the root only, but in complex teeth it exists in considerable quantity, tending to fill in the spaces between the enamel folds of the crown also. Its structure is practically the same as that of bone without Haversian canals, and even these occur where the cement forms a very thick layer. The embedded part of the tooth is attached to the alveolus by a vascular layer of connective tissue, the =alveolar periosteum= (Periosteum alveolare), which constitutes the periosteum at once of tooth and alveolus.

The =blood-supply= to the pulp is derived from the alveolar or dental branches of the internal artery; the =nerve-supply= comes from branches of the trigeminus.

THE TEETH OF THE HORSE

THE PERMANENT TEETH

The formula of the permanent teeth of the horse is:

( 3 1 3 or 4 3 ) 2 ( I - C - P —————— M - ) = 40 or 42 ( 3 1 3 3 )

FIG. 244.—UPPER TEETH OF HORSE, ABOUT FOUR AND ONE-HALF YEARS OLD.

I^1, I^2, I^3, Incisors; C, canine; P^1, P^2, P^3, P^4, premolars; M^1, M^2, M^3, molars. ]

FIG. 245.—LOWER TEETH OF HORSE, FOUR YEARS OF AGE.

I^1, I^2, First and second permanent incisors; Di^3, third deciduous incisor. The cheek teeth are numbered according to popular usage. ]

In the mare the canines usually are very small and do not erupt, reducing the number to 36 or 38.