56 AN INTRODUCTION TO ZOOLOGY 



relatively to the size of the animal, and the amount of excreta is 

 also greater in the former. 



We have seen then that the alimentary canal is a long tube, 

 whose various regions are modified for different purposes. With it 

 are connected glands, which arise in embryonic life as outgrowths 

 from its walls, each of which produces a specific substance or sub- 

 stances, termed enzymes, playing definite parts in the splitting up of 

 certain colloidal compounds. The whole system acts together for 

 the digestion and absorption of the food, in order that it can be 

 utilised by the body. 



Respiratory System. 



Attention has already been directed to the slit-like opening 

 at the back of the floor of the mouth, known as the glottis. It is 

 situated upon a slight median elevation lying between the posterior 

 cornua of the hyoid plate, and it leads into a space, the laryngo- 

 tracheal chamber, or, more briefly, the larynx, whose walls are 

 supported by cartilages. The cartilaginous structures are five in 

 number, a complex ring-like cartilage, the cricoid, runs around its 

 median walls, while the roof of it is supported by a pair of semi- 

 lunar cartilages, the arytenoids, and at the middle of the inner edge 

 of each of these is a small cartilage, the pre-arytenoid. It is these 

 two pairs of cartilages that project slightly into the buccal cavity, 

 and between them lies the glottis. Two flat bands of connective 

 tissue, the vocal cords, related to the cartilages, stretch across the 

 laryngo-trachial chamber, leaving between them a somewhat long 

 and narrow opening, the rima glottidis. They can be approximated 

 by means of a special set of muscles, and so enable the frog to croak 

 by expelling the air from the lungs sharply through the reduced 

 opening. The volume of sound thus produced is increased in the 

 male by the presence of two bags, the vocal sacs, in the floor of the 

 mouth, which can be inflated with air and so act as resonators. They 

 are particularly well developed in R. esculenta. The laryngo- 

 tracheal cartilages are provided with a series of muscles, by means of 

 which the glottis can be closed, while food is being swallowed, and 

 opened, and the chamber distended during the taking in of air, and 

 so on. 



The laryngo-tracheal chamber leads directly into the lungs, a 

 pair of dark-coloured, thin- walled sacs lying far forward in the body 

 cavity on its dorsal side near the heart. The lungs, like the digestive 

 glands, arise during early life from the wall of the alimentary canal.' 

 At first they are represented by a single outgrowth from the floor of 

 the pharynx, but as this grows backwards it splits into two. Their 

 original connection with the pharynxes retained as the glottis. 



