30 THE FROG 
plane as the thyrohyoid process, to which it is closely attached; 
at its posterior end it is produced into a median process or spine. 
From near its middle the cricoid cartilage gives rise to a trans- 
verse ventral loop, the tracheal process, which serves as a means 
of attachment for the lungs. The arytenoid cartilages are a pair 
of semi-lunar valves, whose apposed edges form the lips of the 
glottis; they afford attachment to muscles by which the glottis 
may be opened and closed. 
Cut through the transparent membranous ventral wall of the 
larynx and expose the interior. Observe the vocal cords, two 
strong membranes attached to the anterior and posterior walls of 
the larynx; the inner edges are free and may be set in vibration 
by the expulsion of air from the lungs, thus producing the frog’s 
peculiar croaking sound. The vocal apparatus of the male is 
larger than that of the female. 
Cut open one of the lungs by a longitudinal incision, pin it out 
under water in a small dissecting tray, examine it with a lens and 
sketch a small portion of its inner surface. For this purpose a 
lung that has been preserved while slightly inflated is desirable. 
Of what importance in the respiration of the frog is the hyoid 
apparatus? 
X. THe TuHyroi GLaAnps. 
On each side of the hyoid apparatus, between the lateral process 
and the proximal end of the thyrohyoid process, is a small body 
known as the thyroid gland. For the exact location of these 
glands see figure 60 of Holmes’ Biology of the Frog. Examine 
a gland under the low power of the microscope, observing the 
vesicles of which it is mainly composed. Study a prepared sec- 
tion of the thyroid. Observe the layer of epithelial cells lining 
each vesicle; the space between the vesicles is filled with con- 
nective tissue and blood vessels. Sketch a small portion of the 
section under high power. 
XI. THE JNTEGUMENT. 
A. Mount on a slide a small piece of the outer or horny layer 
(stratum corneum) that has been shed from the skin. Such 
pieces may usually be found in the water in which living or 
