240 THE ORGANS 



to the gums are more developed on the lingual than on the labial side. Between 

 the teeth, fibres of this level pass from the cementum of one tooth to that of 

 the next adjacent, here also supporting the free margin of the gum. Still 

 further apically the fibres are grouped in coarser bundles to form the so-called 

 dental ligament, and pass over or into the edge of the alveolus; between the teeth 

 into the edge of the septum or into the cementum of the adjacent tooth. From 

 the dental ligament for about one-third the depth of the root, rather coarse 

 bundles of fibres pass from cementum almost straight to alveolar wall. For 

 most of the remaining two-thirds, the direction of the fibres is away from the 

 apex, the bundles tending to break up and to radiate to a more extended inser- 

 tion into the alveolar bone. The apical fibres also radiate to their insertion into 

 bone. 



Among the connective-tissue fibres are found fixed connective-tissue cells, 

 groups or cords of cells which are apparently epithelial, and, during development 

 and sometimes sparingly later, osteoblasts, osteoclasts, and cementoblasts. 



The fixed connective-tissue cells are described on p. 87. 



The epithelioid cells are mostly arranged in cords which anastomose to form 

 a network. The cells themselves are granular and have oval or round nuclei 

 rich in chromatin. In some places their arrangement resembles tubules, but 

 whether they have a glandular character is not known. 



Osteoblasts and osteoclasts are the same as in developing bone (p. 202). 

 The latter are present wherever absorption is taking place. They are especially 

 numerous during the absorption of the roots of the milk teeth to make way for 

 the permanent set. 



Cementoblasts are cells which lie against the cementum and are analogous 

 to the osteoblasts, from which however they differ morphologically. They are 

 flattened and their protoplasm fills in the spaces between the ends of the fibres 

 so that the cell bodies show the indentations of the fibres lying against them. 

 From the cementoblasts processes extend into the cementum in much the same 

 manner as processes of bone cells extend into their canaliculi. 



Blood-vessels. — The arteries which supply the tooth and peri- 

 dental membrane, enter the apical portion of the latter from the ad- 

 jacent bone of the alveolus (Fig. 138). On entering the membrane 

 the vessels divide into two main sets one of which passes through the 

 foramina of the apex to supply the dental pulp, while the other passes 

 along the outside of the tooth in the peridental membrane which it 

 suppHes (Fig. 138). The vessels in the membrane anastomose with 

 vessels from the bone of the alveolar wall and at the margin of the 

 alveolar cavity with vessels from the gums. From the capillary net- 

 work veins follow the arteries back to the apical portion of the mem- 

 brane and into the bone at the bottom of the alveolus. The arteries 

 to the pulp run mainly through its centre giving off branches which 

 form a capillary network which is especially rich at the periphery of 

 the pulp (Fig. 142). From these capillaries arise veins which pass 



