16 AN INTRODUCTION TO ZOOLOGY 



small holes, the openings of the Eustachian tubes, and if a seeker 

 be pushed into them it will be found to come to the outside of the 

 head at the tympanic membrane. 



The floor of the buccal cavity is bounded by the lower jaw, 

 which contains no teeth, and is covered by the same soft mucous 

 membrane. This jaw is jointed or articulated to the upper jaw in 

 such a way that it is free to move in a vertical direction. The floor 

 itself is soft but supported by a flat plate of gristle or cartilage, the 

 hyoid plate. Most of its space is occupied by the comparatively 

 large fleshy tongue. This, unlike our own, is bifid at the end, and 

 is attached in front and free at the back, so that it is able to be 

 projected a good distance from the mouth. Behind the tongue is a 

 median slit, the glottis, situated on a small oval elevation, the larynx, 

 through which the air gains access to the lungs. Right at the back, 

 where floor and roof come together, the cavity narrows somewhat, 

 and is termed the pharynx. It communicates with a wide slit, 

 the opening of the oesophagus, which leads to the stomach. If the 

 frog is a male, two small openings lying slightly in front and to the 

 side of the larynx will be seen. These lead into the collapsed vocal 

 sacs, which can be inflated while the animal is croaking, and are 

 particularly marked during the breeding season. 



If the frog be pinned out on its back under water the skin 

 can be slit up from the posterior end to the front of the snout, and 

 also a little way along the base of each limb. In doing this it will 

 be noticed that the skin is not attached to the underlying parts, save 

 by one or two bands of thin white substance, the connective tissue 

 septa. These divide up the fairly extensive cavity into a number of 

 smaller cavities which, since they are filled with a watery fluid, 

 lymph, and are situated under the skin, are termed the sub-cutaneous 

 lymph sinuses. A large blood-vessel, the musculo-cutaneous vein, 

 will be seen running in the skin in the region of the arm. 



Beneath the skin will be seen the flesh or muscles forming the 

 outer wall of the body, arranged in sheets and covered by a very 

 thin layer of semi-transparent substance, the fascia. Passing across 

 the underneath side of the head from jaw to jaw is a broad muscle, 

 the Mylo-hyoideus. Posterior to this is a fan-shaped muscle com- 

 posed of five parts on each side running together at the base of the 

 arm ; this is the pectoralis muscle. In the posterior half of the body 

 in the middle line is a thin clear line of connective tissue, the linea 

 alba, through which a blood-vessel, the anterior abdominal vein, 

 shows clearly. On each side of this line up to the pectoralis the 

 muscles are arranged in a series of small rectangles, the recti 

 abdominis. Between the middle parts of the pectoralis muscles and 

 the linea alba is a small plate of cartilage, the sternum or heart bone, 



