238 
NATURE 
[ Fuly 21, 1870 
THE PHYSIOLOGY OF DIGESTION 
I. MASTICATION 
PROPOSE in this and the following papers to give an 
account of the physiology of digestion, or, in other 
words, to describe the operations to which food is sub- 
mitted, and the alterations it undergoes, before it is ab- 
sorbed into the system and becomes adapted to the 
nourishment of the body, 
With this end in view, it will be expedient to divide 
the alimentary or digestive tract into a series of sections, 
each comprising a portion in which certain definite and, 
in general, well-ascertained processes are effected. The 
first of these is clearly the mouth, where the food, pro- 
vided it be not already of a fluid nature, is ground to a 
pulp and mingled with saliva, or, in other words, where it 
undergoes mastication and insalivation. 
In order to prevent the ingestion of substances that, 
from their temperature, hardness, acridity, putrefaction, 
or other chemical or physical properties, are inappropriate 
as articles of food, the mouth, or vestibule of the alimen- 
tary canal, is guarded by three sentinels that, were due 
attention paid to the impressions derived from them, 
would rarely be found to mislead : Touch, possessed to 
an exquisite degree by the ruddy lips and tongue ; Zasée, 
possessed by the tongue and palate; and Syzed/, which, 
though comparatively neglected by man, is constantly 
Fig. 1.—A, vertical; B, horizontal section of a bicuspid tooth ; a, enamel 
of the crown; 4, pulp cavity; c, cement of the fangs ; d, dentine: magni- 
fied three diameters. 
employed by animals as a means of discriminating suitable 
from unsuitable substances. 
The act of mastication is designed to comminute the 
food, and thus to present a larger surface to the action of 
the several digestive fluids, saliva, gastric and intestinal 
juices, &c., as well as to render it more readily capable of 
incorporation with them, Itis more important that it should 
be thoroughly performed in the case of vegetable than of 
animal food, since the latter is usually of a softer and 
more succulent nature, besides being already analogous 
in composition to the body ; whilst the nutritive material 
contained in the former is enclosed in firm cell-walls 
that are slowly dissolved in the act of digestion, and re- 
quires the action of the several fluids to be long continued 
before it is fitted for nutrition; and it is accordingly 
found that the means for effecting such comminution is 
far more complete in the vegetable than in the animal 
feeders. In a teleological point of view it is interesting 
to notice that in the infant living on milk, which does not 
require mastication, no teeth exist till the fourth or fifth 
month. 
Mastication is accomplished by the movements of the 
jaws, the margins of which are very generally armed with 
teeth. Amongst Mammalia the teeth are only absent in 
the whalebone whales, the anteater, manis, and echidna— 
many rows of small, sharp, hard, epidermal spines. situ- 
/ 
ated on the palate and base of the tongue, however, sup- 
plying their place in the last-named animal, whilst the 
two former may be said to live on animal food that is 
already, in proportion to the bulk of their bodies, ex- 
tremely comminuted. Teeth are indeed absent inthe whole 
group of Birds, and in the Chelonia, doubtless in the for- 
mer case on account of their weight, which would interfere 
with flight, but their place is supplied in both by the 
cutting beak and in the latter also by the powerful gizzard. 
They are absent in the toad amongst Amphibia, and in the 
sturgeon, paddlefish, pipefishes, ammocete and amphioxus 
amongst Fishes, but are elsewhere constantly found 
amongst the Vertebrata; they are very frequent also 
amongst the Invertebrata, though many live by sucking 
the juices of the animals on which they subsist. 
In regard to the teeth of man, it need only here be 
mentioned that they are twenty in number in the child, 
and thirty-two in the adult ; that the six front ones, namely, 
the four incisors and two canines in each jaw, are chiefly 
employed for cutting and tearing the food; and the re- 
maining back teeth, including the four premolars and the 
six molars, for pounding and bruising it. The teeth are 
the hardest parts of the skeleton. Four parts may be distin- 
guished in them—(r1) the pulp, which, occupying a cavity (4) 
in their centre, is extremely smallin quantity, and contains 
an artery vein and nerve ; (2) the dentine (d), which forms 
the greater portion of the tooth and confers upon it its 
general configuration ; (3) the enamel (@), which caps the 
Fic. 2.—View of the external muscles of mastication. A, temporal 
muscle ; D, superficial; and c, deep portion of the masseter muscle, 
free or exposed surface of the tooth, or that portion which 
projects beyond the gum; and (4) the cementum (¢), 
which invests the fang. 
The Dentine (¢) is composed of a series of tubes 
traversing a homogeneous matrix, and extending from 
the pulp cavity to the outer limit of the dentine. The 
course of these tubes is undulatory, and they give off nu- 
merous branches as they pass outwards. It is difficult to 
trace their minutest ramifications even with high powers 
of the microscope. ‘Their interior is occupied by delicate 
solid threads called dentinal fibres, which, however, it has 
recently been shown by Neumann are not in immediate 
contact with the matrix, but are separated from it by a 
resisting membrane or dentinal sheath. The presence of 
these fibres accounts for the sensitiveness which it is well 
known the dentine when exposed possesses, whilst there 
can be no doubt they are subservient to the slow processes 
of nutrition which are continuously taking place in even 
the most superficial parts of these hard organs. 
The Enamel (a) is composed of a series of six-sided, 
solid prisms, which contain scarcely more than 2 per 
cent. of organic matter, but consist almost exclusively of 
phosphate and carbonate of lime. In the rabbit, rat, 
squirrel, and all Rodentia, it forms the cutting edge of 
-their chisel-shaped incisor teeth. 
The Cement (¢) is a peculiar kind of nonvascular bone, 
and though small in quantity and comparatively unim- 
