THE DIGESTIVE TRACT. 



153 



FIG. 186. 



fibres ; the canals left within the matrix to maintain the nutrition 

 of the tissue constitute the dentinal tubules, the homologues of 

 the lacunae and canaliculi of bone. 

 With the continued growth the sides 

 of the papilla as well as the apex be- 

 come covered by the layer of newly- 

 formed dentine ; the central part of 

 the dental papilla remains, after all 

 the dentine has been formed, as the 

 pulp-tissue, into which the blood- 

 vessels and nerves grow at a later 

 period. 



At first the newly-formed dentine 

 is soft, the impregnation with lime- 

 salts occurring subsequently ; the 

 layer of the soft, most recently 

 formed matrix is readily distin- 

 guished in stained sections from the 

 older calcified tissue. The cemen- 

 tum, wanting during foetal life, is 

 produced by the alveolar periosteum./ 



Section of developing tooth from cat em- 

 bryo, portion of preceding figure more highly 

 magnified : tn, mesodermic elements consti- 

 tuting pulp-tissue ; /.layer of odontoblasts en- 

 gaged in producing dentine (K) ; a and 6, cells 

 of middle layer, c and d, cells of inner layer 

 of enamel organ ; e, zone of young enamel. 



THE TONGUE. 



The bulk of the tongue is com- 

 posed of variously-disposed bundles 

 of striated fibres of the lingualis, 

 together with those of the accessory muscles, over the unattached 

 surfaces of which the oral mucous membrane is reflected. _..,.. The 

 muscular tissue of the organ is arranged in bundles extending in 

 three planes : (i) vertically and slightly radially (genio-hyoglossus, 

 vertical fibres of lingualis and hyoglossus) ; (2) transversely (trans- 

 verse fibres of lingualis) ; (3) longitudinally (lingualis superior and 

 inferior, and styloglossus). A vertical median partition, the septum 

 lingualae, divides the muscular tissue into two halves ; the inter- 

 fascicular spaces are filled by delicate connective tissue and fat, in 

 which lie embedded numerous small lingual glands. Many of the 

 muscle-fibres find insertion in the deeper layer of the mucosa, into 

 which their sarcolemma fades. Branched striped muscle-fibres are 

 of common occurrence in the tongue. 



The mucous membrane forms the most conspicuous part of the 

 organ. That covering the sides and inferior surfaces of the tongue 

 is thin, containing small papillae and numerous mucous glands : on 

 reaching the superior surface the mucous membrane greatly increases 

 in thickness, and presents additional conspicuous irregularities, the 



