THE DIGESTIVE SYSTEM 67 



incisors have a crown which is broad transversely; later in life 

 the two diameters are about equal; in extreme age the antero- 

 posterior diameter may greatly exceed the transverse. The angle 

 at which the upper and lower incisors meet becomes rapidly less as 

 age advances. The cup of an incisor tooth disappears about three 

 years after the tooth erupts, and the infundibulum from ten to 

 fifteen years after. Both central incisors in the lower jaw lose 

 their cups when the horse is about six years of age, the laterals at 

 seven, and the corners at eight. At nine years the cups in the up- 

 per central incisors disappear. At ten years a yellowish-brown 

 groove appears at the top of the upper corner incisors, and reaches 

 the wearing surface when the horse is twenty-one years old. The 

 infundibulum gradually becomes smaller, nearer the lingual border, 

 and finally disappears. It remains longer in the upper than the 

 lower incisors because of its greater depth; therefore the latter 

 should be observed when determining the age of a horse. 



The salivary glands occur in pairs. They are named the 

 parotid, the submaxillary, and the sublingual. The parotid gland 

 is the largest and is located in front of the wing of the atlas, behind 

 the upright portion of the jaw and below the base of the ear. Its 

 length is about 10 inches and thickness nearly 1 inch. Its secre- 

 tory duct leads from the anterior lower corner of the gland for- 

 ward and downward, along the inside of the mandible, then it 

 turns outward, around the inferior margin of the jaw, finds its 

 way across the face, and finally ends by piercing the cheek oppo- 

 site the third upper cheek tooth. The submaxillary gland lies 

 internal to the parotid in the submaxillary space. It is about 10 

 inches long and is curved with the concavity forward. The sub- 

 maxillary duct is formed by the union of small radicles, and runs 

 forward from the concave border along the floor of the mouth. 

 It empties beneath the tongue opposite the canine tooth. This 

 opening is plainly seen in the living animal, and is commonly 

 referred to as the "barb." The sublingual gland is placed between 

 the tongue and the horizontal part of the mandible. There are 

 about thirty sublingual ducts, which open in a row on each side 

 of the floor of the mouth. The function of the sahvary glands is to 

 secrete saliva. It has been estimated that the horse secretes from 

 72 to 84 pounds of sahva in twenty-four hours. This is nearly 

 ten times as much as the volume of urine voided in a day. An 

 excessive flow of saliva causes slobbering or drooling. It may be 



