xi DIGESTIVE ORGANS 507 



enclosed in a pericardium, on the ventral surface of 

 which latter is an organ known as the thytnus (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 hps, behind which are the incisor teeth (i). 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 communicate with the 

 nasal cavitfes 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 1 , 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 



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