1029 



TECTUS. 



TEETH. 



1030 



Lord Cornwallis and Colonel Kyd planted it in Bengal about the same 

 time. The Calcutta Botanic Garden contains a number of these trees. 

 From the Saharunpore Botanic Garden, in 30 9' N. lat. (where, its 

 bads being covered with scales, it is enabled to resist cold, besides its 

 leaves falling and giving it a season of rest), the tree has been spread 

 along the Doab Canal ; the whole of the intermediate country is 

 suited to its cultivation, and the East India Company have ordered 

 the Malabar forests to be preserved. The most extensive forests are 

 however those extending along the banks of the Irrawady, especially 

 in Pegu. [TEAK, in ARTS AND Sc. Dry.] 



TECTUS. [TBOCHID*.] 



TEESDA'LIA (named after Robert Teesdale), a genus of Plants 

 belonging to the natural order Cruciferce. It has a roundish notched 

 pouch, boat-shaped valves, their backs keeled below, narrowly winged 

 above ; the seeds two in each cell ; the petals either equal or the two 

 outer ones larger. The species are small annual herbs. The flowers 

 usually small and white. 



T. nudicaulit is the T. Iberit of De Candolle. It has unequal petals 

 and numerous leaves spreading on the ground ; the stamens with 

 remarkable scales within, the pouch emarginate. It is found in sandy 

 and gravelly places in England, France, Germany, Denmark, and 

 Sweden. It is the only British species. T. lepidmm is a native of 

 Europe, particularly of Spain. It differs but little in general characters 

 from the former species ; there are two varieties of it. 



TEETH, in the Animal Kingdom, are those hard organs situated in 

 the mouth, by which food is seized by many animals and masticated 

 by others. In the Fishes and Reptiles the teeth are organs of pre- 

 hension only, but in most of the Mammalia they serve both for pre- 

 hension and mastication. The characters of the teeth of the various 

 classes and families of animals are 



given with the descriptions of the 

 animals in this work. We shall 

 here only refer to the general struc- 

 ture of the human tooth, which 

 may be regarded as the type of all 

 dental structures. 



The best method of preparing 

 teeth for microscopic examination 

 is to immerse them in dilute hydro- 

 chloric acid, till their earthy mat- 

 ter ia BO far dissolved that thin 

 transparent slices may be cut from 

 them with a knife ; or, without 

 softening them in acid, to make 

 thin sections in the vertical and 

 other directions with a fine saw, 

 and to reduce these to the neces- 

 sary thinness and transparency by 

 filing them, and polishing them 

 on a hard and smooth whetstone. 

 For general examination, lenses 

 magnifying about 50 diameters are 

 sufficient. 



In such a vertical section of a 

 tooth three distinct substances are 

 seen, namely, the dentine or ivory 

 (fg.l, a a), which forms the greater 

 man, and, aa it were, the mould 

 of the tooth, and which contains 

 the pulp cavity (4) ; the enamel 

 (c c), by which the crown or ex- 

 posed part of the tooth is covered ; 

 and the bone, cement, or crusta 

 petrosa (dd), which forms a thin 



Fig. 1. 



Magnified section of a bicuspid tooth. 



Fig. 2. 



layer around the fang, except at that part at which the vessels enter 

 the pulp, and is continued in a finer and scarcely perceptible layer 

 over the enamel. 



The bone, or cement, has in each animal a minute structure similar 

 to that of which the bones of its skeleton are composed. In man it 

 consists of a basis of homogeneous substance, a 

 compound of cartilage and earthy matter, in 

 which there are minute cavities (fly. 2) with 

 delicate branched canals leading from one to 

 the other. On the walls of these canals and 

 cells the earthy matter is deposited more thickly 

 than in the intermediate spaces, so that when 

 examined by transmitted light they appear black 

 or dark gray. The cavities, or bone-corpuscles, 

 in man are round and oval, and Battened ; most 

 of them are between ^r.th and yjVt! tn of an 

 inch in length, and about one-third as much 

 in breadth and one-sixth as much in thickness. 

 They have somewhat jagged edges, from all 

 parts of which there proceed the fine branching 

 canals, to which the name of calcigerous has 



Microscopic view of 

 bone-corpuscles and 

 calcigeroua canal*. 



been given, and which traverse the homogeneous basis of the bone, and 

 communicate irregularly with one another. The diameter of these 

 cniiala at their largest parts is not more than ,,,',., th of an inch ; 



that of their smaller branches is between j^^tli and S5 W,th of an 

 inch. Their general direction is towards the axis of the tooth, around 

 which the corpuscles are arranged in concentric circles. 



The enamel is composed of solid prisms, or fibres (fig. 3, a a), about 

 ssisi tn f an inoQ thick, set side by side and upright upon the ivory of 

 the crown of the tooth (6). One end of each prism is fixed in a little 

 depression on the rough outer surface of the ivory ; the other, which 

 is somewhat larger, is turned towards the masticating surface of the 

 tooth in the direction ia which the chief external pressure is to be 

 resisted. The course of the prisms is more or less wavy, their curves 

 being for the most part parallel (fig. 4), but sometimes opposed. Most 

 of them extend from the ivory to the surface of the tooth ; and 

 where they do not, small complemental prisms fill up like wedges the 

 vacant spaces. 



Fig. 4. 



Small collection of 

 uuamcl-fibres. 



' View of the arrangement of the enamel-fibres 

 on the crown of an incisor tooth. 



In the perfect state the enamel contains so small a quantity of 

 animal matter, that it cannot be demonstrated to the sight, and the 

 prisms are inseparably consolidated ; but in young teeth it is soft, and 

 may be broken up into its elementary parts. In the early state also 

 it exhibits portions of a membranous animal substance, consisting of 

 the cells in which each of its prisms was formed ; for, as will be pre- 

 sently shown, the earthy matter is deposited in what might be called 

 a set of moulds formed by the primary cells of the enamel membrane, 

 and, as it accumulates, the membrane of the cell is so nearly removed, 

 that in the perfect tooth no portion of it can be discerned. Its former 

 existence however seems to be indicated by fine close-set transverse 

 stria upon each prism of the enamel. 



Fig. 5. 



1, longitudinal section of a canine tooth, showing the three tissues : a t tho 

 dentine or ivory ; 6, the enamel ; c, the cement or dental bone. 



2, dental tubes as seen by a low power. 



. 3, a longitudinal section of dentine, showing a dentinal tube dilated, and 

 sending oft' anastomosing branches. 



4, longitudinal section of dentine, highly magnified : a, the dentinal tubes ) 

 6, granular intertubular structure. 



ii, transverse section of dentine : a, the tubes, showing their paiictcs and 

 area ; 6, the intertubular tissue. 



The dentine is made up of two distinct parts : first, denlinal tubes } 

 secondly, intertubular tissue. The tubes have distinct parietes, equal 

 in thickness to their calibre. In some instances they appear to contain 

 a minute granular matter, but in many, perhaps in the majority of cases, 

 they are perfectly free from solid contents. If a vertical section, pass- 

 ing through the pulp-cavity, be taken for examination, the dental tubes 

 may be traced from their commencement on the surface of the pulp- 



