i 3 THE PHARYNX. 979 
favourite explanation. For several reasons, unnecessary to detail, this view is now dis- 
carded, and a theory which attributes the impelling force to the blood pressure is looked 
upon with more favour, although even this is not altogether satisfactory. (See Tomes’ 
Dental Anatomy, 5th Edition, page 211.) 
MorPHOLOGY OF THE TEETH. 
In most vertebrates below mammals all the teeth are alike in form; such a dentition is said 
to be homodont. In the majority of mammals, on the other hand, the teeth are arranged in 
groups of different size and form ; such a dentition is heterodont. 
Again, mammals have, neglecting exceptional cases, but two functional sets of teeth ; they are 
consequently said to be diphyodont. Most vertebrates below mammals, on the other hand, have 
a continuous succession of teeth throughout life, and hence are said to be polyphyodont. 
Seeing that practically all lower vertebrates are provided with simple conical teeth, the evolu- 
tion of the many-cusped mammalian molar has given rise to much speculation. The jaws of the 
earliest fossil mammals found are furnished with tri-tubercular teeth, the three tubercles being 
placed in an antero-posterior line ; by a rotation of two of the cusps out or in, as the case may 
be (a condition found in certain other fossil skulls), we arrive at a tri-tubercular form, from which 
the transition to an ordinary mammalian molar is not difficult. As to how the tri-tubercular 
tooth arose from the simple cone, two different views are advanced : one, that it was formed by the 
union of several conical teeth as a result of the shortening of the jaw and the crowding of the 
teeth together ; the other, that the single conical tooth developed lateral buds or outgrowths, and 
that these buds growing larger, the tooth assumed the tri-tubercular form. 
The complete or typical mammalian dentition, in its highest development, as in the horse, is 
represented by the following formula: i, #, c. 4, pm. 4, m.$=44. In the dentition of man, there- 
fore, one incisor and two premolars are wanting. Different views are held as to which teeth 
have been suppressed—most probably they are the second incisors, and the first and second or first 
and last premolars. 
In general it may be said that the dentition of the lower races differs from that of the 
higher, in that the dental arches are squarer in front, the teeth larger and more regular, the 
canines stronger, the wisdom teeth better developed, and the cusps on the molars more perfect, in 
the lower than in the more civilised races. However, according to Tomes, the teeth of a savage 
man, if seen in the mouth of a European, would be looked upon as an “exceedingly perfectly- 
formed set of teeth.” 
To express the proportionate size of the crowns of the premolars and molars to that of the 
skull in different races, Flowers compared the distance from the front of the first premolar to the 
back of the last molar, in situ, with the distance from the front of the foramen magnum to the 
naso-frontal suture (basi-nasal length), in the form of a “dental index ”— 
Length of teeth x 100 : 
Aarne) CR eae eee - = Dental index, 
Basi-nasal length 
and by this means he divided the various races into microdont (index 42 to 48, Europeans, 
Egyptians, etc.), mesodont (index 43 to 44, Chinese, American Indians, Negroes, etc.), and macro- 
dont (index 44 and upwards, Australians, Melanesians, etc.). 
THE PEAR YEN 
| The pharynx is the expanded upper portion of the digestive tube which lies 
behind, and communicates with, the mouth, the larynx, and the nasal cavities 
(Fig. 660). It serves for the passage of both air and food, conveying the former to 
the larynx and the latter to the cesophagus. 
It extends from the base of the skull above to the level of the sixth cervical 
vertebra below (Fig. 661), where, opposite the lower border of the cricoid cartilage, 
it passes into the cesophagus; its total length varies from 5 to 54 inches (12°5 
to 14-0 em.). 
When in its natural state within the body, the pharynx is expanded laterally 
and compressed in the opposite direction, so that its anterior and posterior walls 
approach one another, and its sides are reduced to little more than mere borders. 
Although its cavity is much reduced by this approximation of the anterior and 
posterior walls, there is always left, above the orifice of the larynx, a sufficient 
space for the entrance of air to the lungs. Below the laryngeal orifice, on the 
other hand, the anterior and posterior walls are in contact, and the cavity, except 
during the passage of food, is reduced to a transverse slit (Fig. 661). 
As usually seen in the dissecting-room, when distended ‘for dissection, the pharynx is of an 
elongated oval form, tapering rapidly below. This form, however, is due entirely to the arti- 
ficial conditions under which it is placed. 
