260 THE ARTICULATIONS OR JOINTS 
plete, but is very thin on the inner side. The outer wall of the capsule—the — 
external lateral ligament (lig. temporo-mandibulare, Fig. 204)—is divisible into — 
anterior and posterior portions which are attached superiorly to the root tubercle 
and lower border of the zygoma, and 
inferiorly to the outer side and posterior 
e border of the neck of the mandible. The 
5 ee / direction of its fibres is downwards and 
ay backwards. 
Within the capsule there is an inter- 
articular disc or meniscus of fibro-cartilage, 
the discus articularis (lig. 205), which is 
moulded upon the condyle of the mandible 
below, and on the articular surface of the 
temporal bone above. It thus compensates 
for the incongruity between the articular 
surfaces of the two bones. The dise is 
MANDIBLE ~ attached circumferentially to the capsule. 
It is widest in the transverse direction, 
thicker behind than in front, and thinnest 
towards the centre, where it may be per- 
forated. Its anterior margin is intimately associated with the insertion of the 
external pterygoid muscle. : 
A synovial membrane lines each of the compartments into which the joint 
cavity is divided by the meniscus. Asa rule these membranes are separate from 
each other, but they become continuous when the disc is perforated. The upper — 
synovial membrane is larger and more loosely disposed than the lower. 
Situated on the mesial aspect of the joint, but at a short distance from it, and 
quite distinct from the capsule, there is an accessory ligament called the internal _ 
lateral ligament (lig. spheno-mandi- 
> Emenentia articularis 
Upper joint 
Meniscus 
/ 
i gpaoint capsule 
a) 
Mastoid eee 
process 
Styloid process 
Fic. 205.—SecTIoN THROUGH THE ‘TEMPORO- 
MANDIBULAR JOINT. 
ha 
bulare, Fig. 206). Superiorly it 18 — Spheno-mandibular S 
© é or internal Jateral_ Ne + 
attached to the spinous process of the ligament ‘ 
great wing of the sphenoid bone, and 
inferiorly to the lower as well as the 
hinder border or lingula of the  Mylo-hyoid 
inferior dental foramen. It is not an ES all edadg fy 
articular ligament in the true sense, (A , LZ Z 
for instead of being connected with \.# — AY 
the joint, it is developed in the 
tissue surrounding part of Meckel’s 
cartilage. Stylo-mandibular 
ligament 
Portions of the following structures are —_ 
found in the interval between the spheno- Fic. 206.—INTERNAL LATERAL LIGAMENT OF THE 
mandibular ligament and the ascending TEMPORO-MAXILLARY JOINT. 
ramus of the mandible—viz. the external 
pterygoid muscle; internal maxillary vessels; inferior dental vessels and nerve; middle 
meningeal vessels ; auriculo-temporal nerve ; a deep portion of the parotid gland. 
Movements of the Mandible-—The nature of the movements which the lower jaw can 
perform is determined partly by the character of the articular surfaces of the temporo-mandibular 
Joint, and partly by the fact that, while the two joints always act simultaneously, they may 
also, to some extent, perform the same movement alternately. 
When movement takes place through the long or transverse horizontal axis of each joint, 
the mandible may be elevated, as in clenching the teeth, or it may be depressed, as in gaping. 
In the latter movement the condyle leaves the glenoid fossa, and, along with the meniscus, it 
moves forwards until they rest upon the articular eminence. Meantime the chin describes the 
are of a circle, of which the centre or point of least movement corresponds to the position of the 
inferior dental foramen, and thus the structures which enter at that foramen are protected 
against stretching. Coincidently with the forward movement of the condyle, it glides in a 
revolving manner upon the inferior aspect of the meniscus. 
At any stage in the movement of depressing the chin the mandible may be protruded, so 
that the inferior incisor teeth are projected in front of the upper set, a movement which results 
from the condyles of the jaw being drawn forwards upon the articular eminences. A similar 
