454 



ORGAN SYSTEMS OF MAN 



gut, and their secretions are carried to the 

 gut through a duct. It must be remembered 

 that while the digestive apparatus itself has 

 become more complex with the increasing 

 bulk of the animal housing it, the funda- 

 mental principle of digestion remains un- 

 changed from amoeba to man. The sole 

 purpose of digestion is to break down the 

 large food molecules until they become sol- 

 uble and thus transportable to distant parts 

 of the animal body for use by the proto- 

 plasm there. 



Now that the purpose of taking food into 

 the body is understood, a "tour" through 

 the digestive tube will be taken ( Fig. 17-2 ) . 



THE MOUTH 



Food is taken into the oral cavity through 

 the mouth, where it is crushed into smaller 

 particles by the teeth. During this proce- 

 dure it is thoroughly mixed with saliva, a 

 secretion from the three pairs of salivary 

 glands ( submaxillary, sublingual, and paro- 

 tid ) ( Fig. 17-4 ) . The tongue arises from the 



enamel 



dentine 



pulp chamber. 



connective 

 tissue 



bone 



blood vesseh 



Fig. 17-3. A mammalian tooth In sectional view to show 

 its internal structure. A part of the jaw is also shown 

 to indicate how the tooth is fastened to the bone. 



floor of the oral cavity where it functions 

 as a handy organ in moving the food about 

 and pushing it to the back of the mouth 

 when it is to be swallowed. The tongue is 

 not only highly sensitive to chemicals, as 

 has already been pointed out, but also to 

 touch. A small baby always wants to put 



things in its mouth, the apparent purpose 

 being to satisfy its acute sense of touch. The 

 use of the tongue in forming words needs 

 no further comment. The oral cavity is 

 lined with mucosa which secretes mucus, 

 thus keeping the lining moist at all times; 

 this, together with saliva, aids in lubricat- 

 ing the food so that it can slide down the 

 esophagus without too much friction. The 

 teeth are such important structures both 

 from the utilitarian and esthetic points of 

 view that further consideration should be 

 given them. 



The teeth 



It should be recalled that teeth arise from 

 the ectoderm and that they are actually 

 modified scales similar to those found in 

 the shark ( Fig. 14-4 ) . The tooth consists of 

 the crown which protrudes into the oral 

 cavity, the neck, a narrow region where the 

 gum comes into contact with it, and the 

 root which is firmly cemented in a socket 

 in the jaw bone (Fig. 17-3). The crown is 

 covered with hard enamel which affords 

 a good grinding surface. Under this is the 

 dentine, which is softer, and lying at the 

 center is the pulp chamber which contains 

 the nerves and blood vessels. The tooth is 

 porous and is nourished as long as it is alive. 

 Because of its porous nature, a tooth can 

 be anesthetized by simply applying an 

 anesthetic to the crown, a recent method 

 employed by dentists. 



Man has two sets of teeth during his 

 lifetime. The first set, the milk teeth, begin 

 to appear in the first year of life and are * 

 fully formed by the eighth year. Before they 

 are all well established, however, the front 

 ones start falling out because of the pres- 

 sure of the second set coming from under- 

 neath. So, during the first twelve to sixteen 

 years of life the individual is experiencing 

 a continual loss and replacement of his 

 teeth. The second set, when fully formed, 

 is composed of 32 teeth, eight on each side 

 of both the upper and lower jaws. There 

 are two front cutting incisors, one canine 



