structure evidences organization, and being surprisingly varied in different animals, affords important zoological data.

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The development of Teeth has been most carefully studied by Prof. Goodsir. They are appendages to the mucous, as hair and nails are to the cuticular surface, and no part of the true skeleton. In the fœtus, at six weeks, there forms along the edge of the hard palate "the primitive dental groove," at the bottom of which papillæ" spring up and assume the shape of each future tooth. The groove is next partitioned into "follicles" by septa running between the inner and outer alveolar edge, and then the follicles become "saccules," by closing up round the papillæ. The papilla is analogous to that of hair, and is composed of granular matter, which becomes most highly vascular, and at last ossifies from the surface inwards, the dental tubes also forming. It corresponds only to the crown of the future tooth, and all below the neck subsequently ossifies, and thereby forces the crown to protrude through the gum. enamel is developed in successive layers of particles, which explains the cross markings on the rods, and shows its analogy to cuticle, epithelium, or shells of some animals. It will be remembered that the stages have been successively-grooved, papillary, follicular, saccular; and now, finally, the "eruptive" stage begins.

The

Stages of Dental Development.

The following are the dates at which the papillæ of the twenty temporary or deciduous teeth-viz., 2 incisor, 1 canine, 2 molar, in each side of each jaw-appear: anterior molar, 7th week; canine, 8th; incisor, 9th; and posterior molar, 10th. Even at this early period the

development of the permanent teeth is being provided for, by the mucous membrane being grooved into what Goodsir calls the "cavities of reserve.' The deciduous teeth cut through the gum rather earlier in the lower than upper jaw, and about the following dates of infancy:

The

This completes the first dentition, an epoch often marked by convulsive and other reflex nervous diseases. permanent teeth are being gradually ossified between birth and the third year, and come to lie at the inner edge of the milk-teeth, but much deeper. The fangs of these are absorbed and their crowns thrown off by the permanent teeth under them. The dates of this second dentition are:

These dates, except the last, are very constant, affording proof of the age of children; and Saunders has proposed them as a test of their capability for factory work.

In man the movements of the lower jaw are most varied, combining, as in his teeth and food, the characters of both carnivora and herbivora. Depression is accomplished by the anterior belly of the digastricwhich, in common with all masticating muscles, is sup

plied by the motor part of the trifacial nerve-and perhaps the mylohyoid, genio-hyoid, and genio-hyoglossus. Elevation, by the temporal, masseter, and internal pterygoid. Protrusion, by the anterior fibres of the temporal, the superficial part of the masseter, and the internal pterygoid. Retraction, by the posterior fibres of the temporal and the deep part of the masseter. Lateral motion, by the alternating action of the pterygoids, especially the external.

Insalivation occurs simultaneously with mastication, and thoroughly incorporates with the food the fluids from the parotid, submaxillary, sublingual, and the buccal glands, which are scattered over the cheeks, lips, palate, and back of pharynx. The structure of these is best studied by tracing the single excretory duct as it divides into most minute and numerous branches, ending in cœca and lined by epithelium. The cœca are surrounded by blood-vessels, and are grouped together by areolar

tissue, which condenses on the surface into a capsule. Salivary glands are very simple in the foetus and many lower vertebrata, among all of whom they exist-save fishes, the sea-water with them fulfilling some of the

uses of saliva. Some fishes, however, as the carp, are said to ruminate, and a secretion like saliva is poured out by the pharynx. In the horse, cow, and some other animals, each side of the jaw and salivary glands are used alternately. Its flow is promoted by the motions of the jaw-probably, also, the pressure of the stylomaxillary ligament alternately on the parotid and submaxillary, and remarkably by nervous influence. Every one knows that the sight, or even thought of food, will make the teeth water."

A good deal of saliva is poured out even after the food has been swallowed, the food in stomach exciting reflex action; so that, although "bolting" is injurious, the action of saliva takes place in stomach, the gastric juice not interfering with it, as once supposed. Lunatics and others fed through oesophageal tubes have digested normally. The daily amount is said to be 48 oz. by Bidder and Schmidt.

Saliva is a viscid fluid with a bluish opalescent tint, from epithelial particles, clearing as they subside, and often frothy, its mucus entangling air. Its specific gravity is 1007-9. It is alkaline in proportion, it is said, to the acidity of the gastric juice at the time, but the mucus of mouth during fasting gives it an acid reaction, which has also been remarked during inflammatory diseases. Frerichs found it contained,

A trace of iron can be usually detected. The active principle salivin, or ptyalin, is a yellowish white, nearly

solid matter-having alone the smell of saliva, soluble in ether, alcohol, and slightly so in water-and differing from albumen in being thrown down by acetate of lead and nitrate of silver, not by heat or nitric acid. It holds lime combined with it, and thus saliva, like lime-water, becomes turbid on exposure to air. There is none of it in the saliva of carnivora or sucking animals, as there is no starch in flesh or milk to be converted into sugar. Sulphocyanide of potassium is a salt peculiar to saliva, and recognisable by giving a blood-red tint with persalts of iron. It is absent during ptyalism, owing to dilution, and increased when sulphur is taken internally. The uses of saliva are mechanical or chemical. It reduces the food to a soft, slippery, and easily swallowed mass; it entangles air, and, by soaking food, lets the gastric juice more readily penetrate the bolus. But its main function is, by its ferment, salivin, to saccharize starch. If equal parts of saliva and decoction of starch, made with distilled water, and shown by Trommer's test to contain no sugar, be mixed, the latter loses its jelly-like appearance, produces no blue with iodine, but indicates sugar by its tests as fast as the experiment can be performed. Saliva also breaks the capsules of the starch grains. Wright states that symptoms like hydrophobia may be produced by injecting even pure saliva into blood-vessels of an animal. Salivary calculi and “tartar" consists of 20 per cent. of organic matter, and earthy phosphates, or, according to Wright, 80 per cent. of carbonate of lime, which salt, however, I found wholly absent in one instance.

Deglutition is performed by the mouth, tongue, palate, pharynx, and oesophagus, organs represented on the following page, and is aided by the thick lubricating mucus secreted by the tonsils-racemose glands between the arches of the palate. The act was divided by Magendie into three stages-1. The bolus is placed on the dorsum of the tongue, made hollow to