Page:Encyclopædia Britannica, Ninth Edition, v. 7.djvu/251

233 DIGESTIVE ORGANS 233 incisors round and fusiform, in the lower laterally com pressed, and sometimes marked by a longitudinal groove. Although the human incisors are, as the name implies, cutting, chisel-shaped teeth, in many mammals the incisors are greatly modified in form, as for example in the tusks of the elephant. The determination of the incisor teeth does not depend, therefore, on their form, but on their position in the jaws. The name incisor is given to all the teeth situated in the pre-maxillary portion of the upper jaw, and in the anterior end of the lower jaw, whatever their shape may be. The canine or unicuspid teeth, four in number, one on each side of the mesial plane of each jaw, are placed next the lateral incisors. They are bigger than the incisor teeth, and the upper canines, which are sometimes called the eye- teeth, are larger than the lower ; the fangs of the upper canines are lodged in deep sockets in the superior maxillse, which extend towards the floor of each orbit. The crowns of these teeth are thick and conical ; the fangs are long, single, conical, compressed on the sides where they are marked by a shallow groove. In many mammals these teeth are developed into large projecting tusks. The premolar or bicuspid teeth, eight in number, two on each side of the mesial plane of each jaw, lie imme diately behind the canines, and the upper bicuspids are somewhat larger than the lower. The crown is quadri lateral in form, and convex both on the inner and outer surfaces. It possesses two cusps, of which the outer or labial is larger and more projecting than the inner, palatal, or lingual cusp. The fangs of the upper bicuspids are single and laterally compressed, often bifid at the point into an outer and inner segment ; in the lower bicuspids the fangs are rounded, and taper to a single point. The molar or multicuspid teeth, twelve in number, are placed three on each side of the mesial plane of each jaw. They are the most posterior teeth, are the largest of the series, and as a rule decrease in size from the first to the last ; the crowns of the lower molars are somewhat bigger than those of the upper molars. The last molar tooth does not erupt until the end of puberty, and is called dens sapientice, or wisdom tooth. The crowns are broad, quadrilateral, and convex both on the inner and outer sur faces. The first and second upper molars have four cusps projecting from the angles of the grinding or masticating surface, and an oblique ridge often connects the large anterior internal cusp with the posterior external cusp in the upper wisdom teeth, the two inner or palatal cusps are frequently conjoined. The first lower molar has five cusps, the fifth being interposed between the two posterior cusps ; in the second lower molar the fifth cusp is usually absent, or only rudimentary in size, but in the lower wisdom tooth it is often present. The fangs of the first and second upper molars are three in number, and divergent ; two on the outer or buccal side, one on the inner or palatal side ; in the upper wisdom the fangs are frequently partially conjoined, though triiid at the point. The fangs of the first and second lower molars are two in number, an anterior and a posterior, of which the anterior is the larger ; they usually curve backwards in the jaw ; in the lower wisdom the fangs are usually conjoined, but bifid at the point. The crowns of all the teeth become more or less flattened by use, so that the incisors lose their sharp cutting edge, and the cusps of the premolars and molars are worn away. The temporary or milk teeth are smaller than the per manent teeth. They are more constricted at the neck, where the crown joins the fang, especially in the milk molars, the fangs of which also diverge more widely than in the permanent set. The second temporary molar is bigger than the first. The crown of the first upper molar has three cusps, two buccal, one palatal ; that of the second four cusps. The crown of the first lower molar has four cusps ; that of the second five, three of which are buccal, two lingual. The temporary teeth lie more vertically in the jaws than the permanent. The alveolus, or socket for the lodgment of the single fanged teeth, is a single socket; in the multi-fanged teeth, the socket is divided into two or three compartments, according to the number of the fangs. The socket is lined by the alveolo-dental periosteum, which is continuous at the mouth of the socket with the periosteal covering of the jaw, and with the deeper fibrous tissue of the gum, where it embraces the neck of the tooth. The alveolo-dental periosteum is formed of retiform connective tissue, on the one hand connected with the surface of the cement, on the other with the rrore fibrous periosteum lining the bony wall of the socket (fig. 15), It is vascular, its vessels being continuous with those of the gum, the pulp-vessels, and the bone. It receives nerves from those going to the pulp. The fang fits accurately in the socket, and through a hole at the tip of the fang the blood-vessels and nerves of the tooth pass into the pulp-cavity of the tooth. Structure of the Teeth. Each tooth is composed of the following hard structures dentine, enamel, and cement or crusta petrosa ; occasionally other substances, named osteo- dentine or vasodentine, aro present. In a tooth which has been macerated, an empty space exists in its interior, called the pulp-cavity, which opens externally through the hole at the tip of the fang ; but in a living tooth this cavity contains a soft, sensitive substance named the pulp. The Dentine, or Ivory, makes up the greater part of each Dentir.e. tooth ; it is situated both in the crown, where it is covered by the enamel, and in the fang, where it is invested by the crusta petrosa ; whilst the pulp cavity in the centre of the tooth is a cavity in the dentine. The dentine is composed of an intimate admixture of earthy and animal matter in the proportion of 28 of the animal to 72 of the earthy. The animal matter is resolved on boiling into gelatine ; the earthy matter consists mostly of salts of lime. If thin slices through the Flo 13 _ Transverscsect : nnthroughth 3 dentine of a macerated tooth crown of a tooth - P&amp;gt; P UI I&amp;gt; cavity; &amp;lt;/, . ... n dentine; f. enamel. be examined microscopically, it will be seen to consist of a hard, dense, yellowish- white, translucent matrix, penetrated by minute canals, called dentine tubes. The dentine tubes commence at the pulp cavity, on the wall of which they open with distinct orifices. They radiate in a sinuous manner from the pulp cavity through the thickness of the dentine, and terminate by dividing into several minute branches ; this division takes place in the crown of the tooth immediately under the enamel, and in the fang of the tooth immediately under the crusta petrosa. In their course the dentine tubes branch more than once in a dichotomous manner, and give off numbers of extremely minute collateral branches. The transverse diameter of the dentine tubes near the pulp cavity is - 4 .^Q th inch, but that of their terminal branches is much more minute. If the dentine be examined in a fresh tooth, the tubes will be seen to be occupied by soft, delicate, thread-like prolongations of the pulp. The passage of processes of the pulp into the dentine tubes was first seen by Owen in the examination of the tusk of an elephant ; but the soft con tents of the dentine tubes have been made the subject of special investigation by J. Tomes in the human and other mammalian teeth, and have been named the dentinal fibrils. la sections through the dentine of dried teeth, it is net uncommon to find, near its periphery, irregular, black VII. 30