278 THE ALIMENTARY TRACT, ETC. 



dilatations. This arrangement was first 1 described by Langer, and 

 later by Schobl. The dilatations are placed quite irregularly on the 

 capillaries, sometimes only on one side, sometimes on both sides of 

 the vessel ; a slight constriction occurs at the neck of each dilatation, 

 where it opens into the capillary *. The nerves of the mucous 

 membrane underneath the orbits have nerve-cells attached to them 

 (Stirling and Macdonald, page 170). The lymphatics are very 

 numerous, and for the most part follow the course of the blood- 

 vessels to their finest twigs, beyond which they follow an indepen- 

 dent course (Langer).] 



(2) The teeth are wanting in the lower jaw ; on the roof of the 

 mouth they are found in two situations : as a single row in the 

 groove within the lip, and a group on each vomer. Each tooth 

 has the form of an elongated cone, which in the case of the 

 jaw-teeth is curved, in that of the vomerine teeth straight. The 

 teeth are fixed to the jaw so as to project upwards and slightly in- 

 wards, those at the front of the mouth projecting backwards, and 

 those on the vomers project backwards. Each tooth has a small, 

 sharp projection, or secondary crown on its outer surface (Fig. 181), 

 placed near the mucous membrane. The teeth are fixed to the 

 bones by means of Crusta petrosa, which is again united to the bones 

 of the mandible by a matrix of spongy bone, continuous for the 

 several teeth. Between the tooth and the mucous membrane is a 

 layer of flattened epithelium (H ), which extends more deeply on the 

 inner side of the tooth than on the outer ; the layer is continuous 

 with the epithelium of the mouth, and is two to three cells in 

 thickness. Each tooth has fine longitudinal grooves on the basal 

 part of its outer surface, and consists of two parts, a crown and 

 a root. The teeth possess large cavities, the wall being thin and 

 almost of even thickness, except on the inner surface of the basal 

 portion of the root, where the wall is wanting, and so forms a 

 large aperture to the root. 



The general skeleton of the tooth is formed of dentine, this is 

 covered on the crown by a layer of enamel, and the Ciiticula dent-is 

 on the socket by a layer of Cruda petrosa. 



1 Beale, Phil. Trans., 1863, p. 153, shows dilatations in his drawing, but makes no 

 reference to them in the text. 



2 The vascular supply of the mucous membrane of the dorsal surface of the mouth 

 and oesophagus is shown (after Schobl) in Plate II, Fig. 180, I. Arteries red, veins 

 blue. II. in the same figure represents the dilatations on the capillaries ; after 

 Schobl. 



