xi DIGESTIVE ORGANS 507 



on the ventral surface of which latter is an organ known as 

 the thymus (see p. 447) : the gullet and main blood-vessels 

 also pass through the thorax. The abdomen encloses the 

 greater part of the enteric canal, together with the liver 

 and pancreas, the spleen, and the urinogenital organs. 

 The diaphragm is convex on its anterior side, towards 

 the thorax : it consists of a central, thin, tendinous portion 

 into which radial muscles are inserted. These arise 

 from the vertebral column and posterior ribs, and are 

 especially strong on the dorsal side, where they form two 

 bands known as the pillars of the diaphragm. When the 

 muscles contract, the diaphragm is made flatter, and thus 

 the thoracic cavity is enlarged. 



Digestive organs. The mouth-cavity (Fig. 135) is large, 

 and the small gape is bounded by upper and lower lips, be- 

 hind which are the incisor teeth (/). On either side of the 

 cavity are the borders of the upper and lower jaws from 

 which the cheek-teeth project : these are separated from 

 the incisors by a considerable interval or diastema. Close 

 behind the upper incisors are a pair of very small openings 

 leading into the naso-palatine canals (n.p.c), which communi- 

 cate with the nasal cavities but must not be confounded 

 with the internal nostrils. The roof of the oral cavity is 

 formed by the palate, the anterior part of which, or hard 

 palate (h. p\ is transversely ridged and partly supported 

 by bone (h. p' , p. 494) ; while the posterior part, or soft 

 palate (s. p] is smooth, its hinder, free edge forming a 

 pendulous flap, the velum palati, on either side of which 

 is an organ known as the tonsil, consisting of connective 

 and lymphoid tissue and resembling the " lymphatic glands " 

 (compare p. 515) which occur along the lymph-trunks ; it 

 has the form of a small pit with a broad papilla on its 



