THE LARYNX. 



1815 



Development and Growth. — The thyroid, probably formed from the fourth 

 and fifth branchial arches, is originally rounded in front, the angle becoming promi- 

 nent at puberty, when the great increase in size in the male and the greater promi- 

 nence occur. A slight strip of cartilage, separate from the rest, is found in the angle 

 in early childhood ; subsequently it becomes less and less distinct. 



Variations. — It is not rare to find a foramen near the upper outer angle, a little below the 

 superior tubercle, which transmits the superior laryngeal artery and exceptionally some fibres 

 of the external branch of the superior laryngeal nerve. Assuming that the thyroid is developed 

 as above stated, the foramen represents a cleft between the fourth and fifth branchial bars. It is 

 common for one of the superior horns to be shorter than the other, and not very rare for one to 

 be absent. Our experience agrees with that of others in finding the absence more common on 

 the left side. 



Cartilaeo triticea 



Thyro-hyoid 

 membrane, left 

 half 



Cartilage 

 of Santorini 



Posterior 

 crico-arytenoid 

 ligament 



Cricoid 



cartilage 



Hyoid 

 bone 



Arytenoid 

 cartilage 



Joints and Ligaments connecting the Thyroid with the Cricoid Car- 

 tilage and with the Hyoid Bone. — ^The crico-thyroid joints, between the 

 lower articular facets of the cricoid and the inferior horns of the thyroid, are very 

 indefinitely shaped. The facet of the thyroid is on the inner side of the inferior horn, 

 and is nearly plane, but either par- 

 ticipant of the joint may be the Fig. 1541. 

 contained one. The capsule is Epigiouis 

 lax, although somewhat strength- 

 ened by two by no means con- 

 stant ligamentous bands. An an- 

 terior one extends downward and 

 forward from the front of the 

 lower horn ; a posterior one ex- 

 tends upward and backward from 

 the back of the same. The motion 

 IS usually described as rotation on 

 a transverse axis passing through 

 both joints, but in fact a great deal 

 of irregular sliding is possible. 



The crico-thyroid mem- 

 brane, although connecting the 

 cartilages in front, has no direct 

 attachment to the thyroid at the 

 sides, and consists of a central 

 anterior and a lateral part. The 

 anterior party also known as the 

 conoid ligament, is triangular in 

 shape, with its base attached to 

 the upper edge of the cricoid car- 

 tilage and its truncated apex to 



the lower border of the thyroid. This is the strongest part of the membrane, con- 

 taining considerable elastic tissue, and closes the middle of the space between the two 

 cartilages. It is pierced by several small holes for blood-vessels, and is crossed 

 superficially by the crico-thyroid artery. The lateral part (Fig. 1544), while directly 

 continuous with the anterior and attached below to the upper border of the anterior 

 arch of the cricoid cartilage, is thin and membranous, and on each side extends 

 upward and inward beneath the lower border of the thyroid ala without being at- 

 tached. The upper border of this part of the membrane becomes directly blended 

 and continuous with the inferior thyro-arytenoid ligament, the latter being practically 

 the thickened and free superior border of the crico-thyroid membrane, which in this 

 sense becomes the supporting framework for the true vocal cord. The lateral crico- 

 arytenoid and thyro-arytenoid muscles intervene between the thyroid ala and the 

 lateral part of the membrane. The inner surface of the latter is covered by the 

 laryngeal mucous membrane. 



The thyro-hyoid ligament or membrane is one continuous sheet of fibrous 

 tissue, the posterior borders of which are thickened as they extend between the supe- 



Thyroid 

 cartilage 



Posterior 

 crico-thyroid 

 ligament 



Trachea 



Cartilages of larynx united by their ligaments ; right half of thyro 

 hyoid membrane has been removed ; postero-lateral aspect. 



