42 COMPARATIVE ANATOMY 



hoop, with various processes, called the cricoid; a pair of 

 crescentic cartilages, placed in front of it, called the arytenoids ; 

 and a smaller pair, connected with these, called the pre- 

 arytenoids. These cartilages are moved by special muscles, 

 and they bear a pair of flat bands of connective tissue — the true 

 vocal cords — lying parallel to one another, with a slit-like space 

 between — the rima glottidis. The cartilages and muscles are 

 so disposed that the edges of the vocal cords forming the sides 

 of the rima glottidis may be approximated or separated, whilst 

 the cords themselves are tightened or slackened. The cords 

 are thrown into vibration by the passage of air over their 

 tightened and approximated edges, and thus a sound — the well- 

 known croak of the frog — is produced. The male frog croaks 

 much more loudly than the female, and its laryngeal cartilages 

 are larger, thicker, and stronger. The croak of the male is 

 further intensified by a pair of resonators, in the form of vocal 

 sacs, which open one in each corner of the floor of the hinder 

 part of the mouth. When the animal croaks the sacs are 

 inflated, and bulge out under the angles of the mouth. 



- The lungs take their origin from the posterior part of the 

 laryngeal tube, their openings being kept extended by a pair 

 of curved cartilaginous processes of the cricoid. Each lung 

 starts as a contracted tube from the larynx, and then swells out 

 into a pear-shaped bag which lies in the body cavity dorsal to 

 the liver. The lungs have highly elastic walls, which, by their 

 contraction, expel the air in a dead frog and leave the lungs 

 shrunken and collapsed. They are easily inflated by means of 

 a blow-pipe inserted in the glottis. The internal structure of 

 the lungs is somewhat complicated. The cavity of the sac, 

 especially its upper part, is broken up into chambers by a 

 number of folds or partitions passing into the interior. These 

 chambers are further subdivided by secondary folds starting 

 from their walls, and the smaller chambers so formed may 

 again be subdivided in a similar manner into minute chambers 

 termed alveoli. The whole of the upper part of the lung thus 

 assumes a sort of honeycomb structure, but in its lower 

 pointed end the chambers are less developed, and at the 

 extreme apex are absent altogether. The honeycomb arrange- 

 ment provides a large surface for exposure to the air contained 

 in the lungs ; and, as the partition walls are exceedingly thin, the 

 blood which circulates through them is brought so close to the 



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