The Liver

In invertebrate animals this organ is always ultimately resolvable into caecal tubes, the ends of the hepatic ducts, which are lined with an epithelium, and not reticulated and it has no receptacle for the bile. In most Vertebrata the ends of the hepatic ducts have not been satisfactorily traced, nor is it certain that the immense proportional mass of hepatic corpuscles is contained in tubes continuous with them; if such be the case, the tubes must be reticulated. The ducts of the vertebrate liver very frequently pour the bile, directly or indirectly, into a receptacle, the gall-bladder. Amphioxus stands alone among vertebrated animals, in having a caecal diverticulum of the intestine for a liver.

The Teeth
Teeth, in Mollusca and Annulosa, are always "ecderonic," cuticular, or epithelial structures. In Vertebrata true teeth are invariably "enderonic," or developed, not from the epithelium of the mucous membrane of the alimentary canal, but from a layer between this and the vascular deep substance of the enderon, which answers to the dermis in the integument. The horny "teeth" of the Lampreys, and of Ornithorhynchus, appear to be ecderonic structures, homologous with the "baleen" of the Cetacea, with the palatal plates of the Sirenia, or the beaks of Birds and Reptiles, and not with true teeth.

The dense calcified tissue called dentine, characterized by the close-set parallel tubuli which radiate through it, branching as they go, constitutes the chief mass of true teeth; but the dentine may be coated with ordinary bony tissue, which then receives the name of cementum, and its crown may be capped with imperforate, prismatically fibrous, enamel.

The teeth are moulded upon papillae of the mucous membrane, which may be exposed, but are more usually sunk in a fold or pit, the roof of which may close in so as to form a dental sac. And there may be one set of teeth, or several; the sacs of the new teeth, in the latter case, being developed either as diverticula of the old ones, or independently of them.

In the majority of the Mammalia the teeth are limited in number, as well as definite in their forms and their mode of succession. There are two sets of teeth, forming a first, deciduous, or milk dentition, and a second, or permanent dentition. The deciduous dentition, when most completely developed, consists of incisor, canine, and molar teeth. The incisors are distinguished from the rest by the lodgment of the upper set in the premaxillae, and the correspondence of the lower set with the upper. Their number and form vary. The distinction between canines and molars is one of form and position in regard to the remaining teeth; the most anterior of the teeth behind the premaxillo - maxillary suture, if it is sharp and projecting, receiving the name of canine. There are never more than four canines. The other teeth are molars, and ordinarily do not exceed four upon each side, above and below. What is called a dental formula is a convenient combination of letters and figures for making the number and disposition of the teeth obvious. Thus, let di, dc, dm, represent, respectively, the deciduous or milk set of incisors, canines, and molars. Then, by placing after each of these symbols figures arranged so as to show the number of the teeth of the kind symbolized, on each side of each jaw, we shall have the dental formula of a given animal. The dental formula of a child over two years of age is thus -

-di	2.2	dc.	1-1	dm.	2.2	=20
	2.2		1-1		2.2

which means that the child should have two incisors, one canine, and two molars on each side of each jaw.

The neck of the sac of each deciduous tooth gives off a diverticulum, in which one of the permanent teeth is developed; as it grows, it causes the absorption of the fang of the corresponding deciduous tooth, which thus becomes shed, and is replaced from below by the permanent tooth. The same letters, but without the prefix d, are used for the permanent incisors and canines; but the permanent teeth, which replace the deciduous molars, are called premolars, and have the symbol pm. Furthermore, three or, it may be, four permanent grinding teeth, on each side of each jaw, are developed altogether behind the milk molars, and thus come tinto place without replacing any other tooth from below. These are called molars, and have the symbol m. Thus the formula of the permanent dentition in Man is written:

-i	2.2	c.	1-1	dm.	2.2	m.	3.3	=32;
	2.2		1-1		2.2		3.3

there bemg two mcisors, one canine, two premolars, and three molars on each side above and below. It is a rule of very general application among the Mammalia, that the most anterior molar comes into place and use before the deciduous molars are shed. Hence, when the hindermost premolar, which immediately precedes the first molar, comes into use by the shedding of the last milk molar, the crown of the first molar is already a little ground dovm; and this excess of wear of the first molar over the adjacent premolar long remains obvious. The fact that, in the permanent dentition, the last premolar is less worn than the first molar which immediately follows it, is often a valuable aid in distinguishing the premolar from the molar series.

No vertebrate animal has teeth in any part of the alimentary canal save the mouth and pharynx - except a snake (Rachiodon), which has a series of what must be termed teeth, formed by the projection of the inferior spinous processes of numerous anterior vertebrae into the oesophagus. And, in the highest Vertebrata, teeth are confined to the pre- maxillae, maxillae, and mandible.