242 THE DIGESTIVE ORGANS. 



nuclei and two or three processes extending into the canaliculi of 

 the dentin, forming here the dentinal fibers already described. As 

 a rule, the odontoblasts also send a single fiber into the pulp. 

 These may intertwine and give rise to a network within its sub- 

 stance. 



Peridental Membrane, Alveolar Periosteum. The tooth is joined 

 to the alveolus by a fibrous tissue membrane, the peridental mem- 

 brane or alveolar periosteum, which represents the periosteum of 

 the alveolus and the cementum of the tooth. This consists of 

 bundles of connective tissue (elastic fibers are here absent) directly 

 continuous with Sharpey's fibers in the cementum and the alveolus. 

 Between these coarser bundles of fibers, which have a direction 

 nearly horizontal in the upper portions of the peridental membrane 

 and incline toward the lower end of the tooth in its lower portion, 

 there is found a looser connective tissue, containing numerous 

 nerve-fibers, blood-vessels, and peculiar masses of epithelial cells 

 representing the remains of the enamel organs, to be described 

 later. These epithelial remains have by some observers been 

 regarded as glandular in nature; further observation is, however, 

 needed before this can be accepted as proved. At the apex of the 

 root there is found a less dense connective tissue, continuous with 

 that of the tooth pulp. At the neck of the tooth the peridental 

 membrane disappears in the submucosa of the gum. 



The blood-vessels of the teeth have been fully described by 

 Lepowsky, who has studied them in a number of mammals, and in 

 man in embryos and in full development ; his account is here fol- 

 lowed. The artery, accompanied by the veins, enters through the 

 apical foramen, passes up through the pulp, dividing into branches 

 as it reaches the upper portion of the pulp cavity; these branches 

 are spread out fan-shaped and after further division and the forma- 

 tion of capillaries, end in capillaries which are situated between the 

 layer of odontoblasts and the dentin, forming here a capillary plexus 

 which presents narrow meshes, in regions where the odontoblasts 

 are most active. 



There are in all probability no lymphatic vessels in the 

 pulp. 



Numerous medullated nerve-fibers (dendrites of sensory neurones) 

 enter the pulp cavity through the apical foramen. Some of these 

 lose their medullary sheaths soon after entering, or just before 

 entering, the pulp, and divide into long, fine, varicose fibers which 

 interlace to form a loose plexus under the odontoblasts. Other 

 medullated fibers, grouped into small bundles, ascend in the pulp 

 for variable distances ; the nerve-fibers of the bundles then sepa- 

 rate and as single fibers approach the superficial portion of the 

 pulp, and, after losing their medullary sheaths, divide into fine 

 varicose fibers forming under the odontoblasts a plexus continuous 

 with the plexus above mentioned. From the varicose nerve-fibers 



