338 DIGESTIVE SYSTEM OF THE HORSE 



2. Hyo-glossus. — This is a wide, flat muscle, somewhat thicker than the pre- 

 ceding. It hes 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 oliliquely 

 forward and upward, and for the most part turn toward tlie 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 l)aseo-, kerato-, and chondro-glossus of human anatomy.) 



3. Genio-glossus. — This is a fan-shaped muscle, which lies parallel to the me- 

 dian 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 fil)ers 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 fillers retract the tip of the 

 tongue. 



Vessels and Nerves. — The arteries of the tongue are the lingual and sub- 

 lingual 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 papillae. 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. Thecheekteeth (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 l)y permanent successors, and a posterior 

 series, the molars, which aj^pear only as permanent teeth without deciduous pre- 

 decessors. 



As the teeth of the two sides of the jaws are alike in numljer and character 

 (in normal cases), the complete dentition may be briefly indicated V)y a dental 

 formula such as the following: 



/ 2 1 2 :3\ 

 2 I I - C - P - M - I = 32. 

 V 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 upi^er 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 



