394 DIGESTIVE SYSTEM OF THE HORSE 



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. ^ 



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 

 layer of fat and areolar tissue. It arises from the medial surface of the ramus 

 of the mandible just behind the symphysis. From the tendon the fibers pass in 

 a radiating manner, some curving forward to the tip, others pass 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 pro- 

 trude the tongue, the middle fibers depress the tongue, and the anterior fibers 

 retract the tip of the tongue. 



In some cases there is a small anomalous muscle which arises by a delicate tendon with the 

 genio-glossus and is attached behind with the genio-hyoideus. 



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

 lingual branches of the external maxillary artery. The veins go to the internal 

 and external maxillary veins. The lymph vessels go chiefly to the pharyngeal 

 lymph glands. The sensory nerves are the lingual and glosso-pharyngeal, and the 

 muscles are innervated by the hypoglossal nerve. 



THE TEETH IN GENERAL 



The teeth are hard white or yellowish-white structures implanted in the alveoli 

 of the bones of the jaws. Morphologically they are large calcified papillae. Func- 

 tionally they are organs of prehension and mastication, and may serve as weapons. 

 The domesticated mammals have two sets of teeth. The teeth of the first set 

 appear during early life and are known as deciduous or temporary teeth (Dentes 

 decidui),^ since they are replaced during the period of growth by the permanent 

 teeth (Dentes permanentes) . 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, and 

 interrupt the interalveolar space. 



3. The premolar and molar teeth (Dentes prjemolares et molares) form the 

 sides of the dental arch. The premolars form the anterior part of the series; they 

 appear in both sets. The molars appear only in the permanent dentition. The 

 term cheek teeth is used to include both premolars and molars. 



The interval between the incisors and premolars is the interalveolar or inter- 

 dental space. 



As the teeth of the two sides of the jaw 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 



/ 2 1 2 3\ 



( 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 upper and lower jaw 

 respectively in man. 



' It may be possible to recognize in this muscle three parts, which would correspond to 

 the baseo-, kerato-, and chondro-glossus of man. 



^ They are also popularly spoken of as "milk" teeth. 



