THE FROG 29 
contraction of these hearts usually causes a slight pulsation of the 
skin, but it may be seen more distinctly in a freshly-killed speci- 
men from which the skin of this region has been removed with- 
out injury to the hearts. 
IX. THe RESPIRATORY SYSTEM. 
Respiration in the frog is not carried on exclusively by means 
of the lungs; the skin and the mouth cavity are important 
accessory organs of respiration. 
The lungs communicate with the pharynx or posterior part of 
the mouth cavity by means of the glottis, which opens from the 
dorsal surface of a cartilaginous box, the larynx, whose cavity is 
continuous with the cavities of the lungs. 
Returning to the study of your original specimen, remove the 
liver, taking care not to injure the adjacent parts. Remove the 
skin and muscle from the throat region and expose the hyoid 
apparatus, the body of which consists of a thin shield-shaped 
sheet of cartilage, lying just in front of the larynx. This 
cartilage serves for attachment of the tongue and several muscles 
that move the floor of the mouth; a pair of strong bony processes, 
the thyrohyoid processes, extend posteriorly and support the 
larynx which lies between them. After examining a preparation 
of the hyoid, larynx and lungs, remove these organs as follows: 
cut the cesophagus across about 5 mm. behind the base of the 
lungs and, inserting one blade of the scissors in the open end, cut 
forward along the side toward the angle of the jaw and dorsal to 
the root of the lung of that side, taking care not to injure the 
lung. Continue this cut forward through the floor of the mouth 
close to the jaw to a short distance in front of the hyoid appa- 
ratus; make a transverse cut here and a cut similar to the first 
down the other side of the cesophagus. Remove the piece cut out 
and place it, ventral surface up, in a dish of water. Taking care 
not to injure the thyroid glands (see Section X), clear away the 
muscles in case this has not already been done completely, and 
observe the larynx embraced by the thyrohyoid process of the 
hyoid. The skeleton of the larynx is composed of the cricoid and 
the arytenoid cartilages. The cricoid cartilage consists of a 
slender ring surrounding the larynx and lying in nearly the same 
