THE EXTERNAL EAR AND EUSTACHIAN TUBE. 41 
cells. This also disappears, and the vesicle is embedded in the base of the head in 
a region above and between those parts of the basal axis which afterwards are 
transformed into the basi- and pre-sphenoid elements of the sphenoid bone. 
During the period of its formation and separation the ingrowth from the 
stomatodzeum comes into relation posteriorly with a small diverticulum from the 
floor of the fore-brain, which dilates at its lower end to form the posterior or 
cerebral lobe of the pituitary body, whilst its upper part remains as the infundi- 
bulum, the connecting stalk between the pituitary body and the floor of the third 
ventricle of the brain. The anterior or stomatodzal lobe of the pituitary body is 
much larger than the posterior lobe, which it surrounds and conceals both in front 
and at the sides. 
It is evident that in the early stages the pituitary body consists of two 
ectodermal vesicles, the cavity of the posterior: vesicle is continuous with the 
cerebral tube, and that of the anterior vesicle with the cavity of the primitive 
mouth. The cavity of the posterior vesicle is generally obliterated, and though 
nervous structures are for a time developed in its walls they entirely disappear in 
man and are replaced by vascular connective tissue. Occasionally a small part of 
the cavity remains as a minute vesicle lined with columnar ciliated epithelium. 
The cavity of the anterior vesicle persists, it sends out numerous diverticula, 
and is gradually converted into a number of tubular spaces, lined with cubical or 
columnar cells, united together by vascular connective tissue which has grown 
amidst the tubules from the surrounding mesoderm. 
THE EXTERNAL EAR, THE TYMPANIC CAVITY, AND THE 
EUSTACHIAN TUBE. 
The external ear, the tympanic cavity, and the Eustachian tube are all developed 
from the first visceral cleft and its boundaries. The cleft lies between the 
mandibular (first) and the hyoid HB 
(second) visceral arch in the side 
wall of the pharyngeal portion of 
the fore-gut, and, before a neck 
is developed, it extends from 
just beneath the otic vesicle, 
which lies at the side of the hind- 
brain, above, to the pericardial 
region below. The membrane 
which lies at the bottom of the BY ~— 
cleft consists in the early stages 
of ectoderm and entoderm, but 
in a short time a thin layer of 
mesoderm grows between the 
two primary layers, and the 
trilaminar septum is ultimately 
converted into the tympanic 
membrane which separates the 
external from the middle ear. —— ae 
The differentiation of the Fic. 33.TRANSVERSE SECTION THROUGH THE HEAD 
outer part of the cleft is initiated or a Rar Empryo. 
by the appearance of six tuber- Showing the rudiments of the three parts of the ear and their 
: : . relation to the hyo-mandibular cleft. 
cles round its margins, which are PES e TS NEEy oor.) cot napeeaanaee BEA 
3 . BY. Blood-vessels. N. Notochord. 
afterwards transformed into the GM eaties OV. Otic vesicle. 
several parts of the pinna. EM. Ext. auditory meatus. P. Pharynx. 
Two tubercles are formed an- ET. Eustachian tube. tL. Recessus labyrinthi. 
: : : HB. Hind-brain. SC. Semicircular canal. 
teriorly on the mandibular arch, HM. Hyo-mandibular cleft. T. Tympanum. 
one at the upper end of the cleft 
and three posteriorly on the hyoid arch. The two tubercles on the mandibular 
arch are a small lower, the tuberculum tragicum, and a larger upper, the tuberculum 
anterius helicis. The tubercle at the upper end of the cleft is the tuberculum 
