300 



THE MOUTH. 



nniform rapidity, for it gi-ows especially dming the seventh, eighth, and ninth 

 montlis of intra-uterine existence. 



After biith, the thymus, as ah-eady stated, continues to grow until near the end 

 of the second year. According to the observations of Haugstedt and Simon, it 

 appears for a short time after birth to increase in weight not merely absolutely, 

 Ijut even faster than the rest of the system, and during the next period only to 

 keep pace with the increase of the body. After the second year it ceases to gi'ow. 

 and Ijecomes gradually converted by the eighth or twelfth year into a fatty mass, 

 the corpuscles disappearing or becoming developed into the cells of adipose tissue. 

 At pubert,y the thymus is generally reduced to a mere vestige which has entirely 

 lost its original stracture, and consists of brownish tissue occuiiying the upper 

 part of the anterior mediastinum. Occasionally it is still found in good condi- 

 tion at the twentieth year ; but generally only traces of it remain at that time, 

 and these are rarely discoverable beyond the twenty-fifth or thirtieth year. 



The thymus gland presents no difference in the two sexes, and exists, according 

 to Simon, in all animals breathing by lungs. It appears in all to become even- 

 tually transformed into a mass of fat. 



OKGANS OF DIGESTION. 



The ditjestice aj)paratus consists mainly of the alimentary canal, to- 

 gether with various glands of which it receives the secretions. 



The alimentary canal commences at the mouth and terminates at 

 the anus. Its average length is ahout thirty feet, — about five or 

 six times the length of the body. 



The part situated in the head and thorax consists of the organs of 

 mastication, insalivation, and deglutition, and comprises the mouth, 

 with the teeth, and salirarij glands, the pliarijnx, and the msophaniis or 

 gullet. The part contained in the abdomen and pelvis consists of the 

 stomach and tlie small and large intestine. The glands which are most 

 intimately connected with digestion are very numerous small glan- 

 dular organs situated in the mucous membrane of the alimentary canal, 

 and the larger glands, such as the salivary glands, pancreas and liver, 

 whose ducts open on its inner surface. 



THE MOUTH. 



The mouth is included between the lips and the throat. Bounded by 

 the lips, cheeks, tongue, and the hard and soft palate, it communicates 

 behind with the pharynx through an opening called iha fauces (isthmus 

 I'aucium). It is lined throughout by a mucous membrane, which is of 

 a pink rosy hue during life, but pale grey after death, and which pre- 

 sents peculiarities of surface and structure to be noticed hereafter. 



The lips and cheeks are composed externally of skin, and internally of 

 mucous membrane, together with muscles, vessels, and nerves fully 

 dtseribed in otlier parts of this work, some areolar tissue, fat, and 

 numerous small glands. The free border of the lips is protected by a 

 dry mucous membrane, which becomes continuous with the skin, is 

 covered with numerous minute papilke, and is highly sensitive. In 

 some of these pa})illa3 nerve-end-bulbs, a[)proaching in character to 

 tactile corpuscles, are found (see fig. 101 a, p. 148), in others coiled 

 nerve-fibres (KiUliker), On the inner surfoce of each lip, the mucous 

 membrane forms a fold in the middle line, connecting the lip with the 

 gums of the corresponding jaw. These are ilio, fmna ovfncnula of the 

 lips : that of the upper lip is the larger. 



