3<30 DIGESTIVE OEGANS. [SECT. 141. 



the pulp like a columnar epithelium. Further inwards, these cells 

 are arranged less regularly, and at length pass into the vascular tis- 

 sue of the pulp without well defined limits by the medium of shorter 

 and more roundish cells. These cells correspond to the formative 

 cells of the dentine, to be afterwards described, and they furnish the 

 materials for the deposition of dentine on the walls of the dental 

 cavity which may occur in the adult. The vessels of the pulp are 

 extremely numerous, whence its reddish colour. From three to 

 ten small arteries pass into the pulp of a single tooth, which, at 

 length, produce both in the interior of the pulp and upon the 

 surface of the pulp, a somewhat loose net- work of capillaries 

 croo^" to o'oo6" wide, from which the veins then arise ; at the 

 surface these capillaries here and there present evident loops. The 

 tooth-pulp does not appear to possess any lymphatics, but its nerves 

 are extremely well developed. Arising from the well-known nervi 

 dentales, there passes into each fang a trunk, o"03'" to o - 04'" in size, 

 and, together with it, as many as six or even more fine twigs, o*oi'" 

 to o # 02'", which, having nerve-tubes 0'00i6"' to o , oo3'" in diameter, 

 ascend into the cavity without, at first, forming any considerable 

 anastomoses, but giving off separate fibrils, and in the thicker part 

 of the pulp form a richer and richer plexus, with elongated meshes 

 and collections of nerve-tubes, and thus gradually break up into 

 fine primitive fibres, o-ooi'" to o , ooi6 / " in diameter. These primi- 

 tive fibres form very evident loops, but it is uncertain whether these 

 represent the ultimate terminations. 



The gum, gingiva, is that part of the mucous membrane of the 

 oral cavity, which covers the alveolar borders of the jaws, and 

 embraces the necks of the teeth. It is a light red, vascular tissue, 

 which, owing to the hard subjacent parts, is firm to the feel, 

 although it is in reality soft, and which, where it is applied to 

 the teeth themselves, attains a thickness of £'" to ij"', bears 

 tolerably large papilke (o'i5" to o - 3"'), and possesses a pavement 

 epithelium 0'23'" to 0*4'" in thickness between the papillae. I 

 have not been able to find a trace of glands in the gum, and we 

 must be careful not to take for the orifices of glands certain 

 rounded depressions of the epithelium, 0'o8"'to 0"i5'" in diameter, 

 with cornified epithelial cells. 



§ 141. The Development of the Teeth. — The development of 

 the twenty milk teeth begins in the sixth week of foetal life, with 

 the formation of a groove upon the upper and lower maxillary 

 border. From the time of formation of these grooves up to the 



