The Frog — An Amphibious Vertebrate Animal 435 



small cavity) to increase the surface exposed to the air. Thin-walled 

 capillaries line the inner surfaces of the alveoli and permit the exchange 

 of oxygen and carbon dioxide between the air in the lungs and the blood 

 in the circulatory system. The amount of exchange of these gases 

 depends upon the concentration of each on either side of the lung and 

 blood vessel membranes. Air is forced into the lungs through the slit- 

 like glottis by closing the nares and contracting the floor of the mouth. 

 It is expelled from the lungs through the glottis into the mouth cavity 

 by the contraction of the muscles of the body walls. Air may be expelled 

 or drawn into the mouth through the nares by closing the glottis and 

 alternately raising or lowering the floor of the mouth. Sounds may be 

 produced by forcing air back and forth through the glottis (Fig. 212). 



Oxygen unites temporarily with the hemoglobin of the red blood cor- 

 puscles, forming oxyhemoglobin. The latter carries oxygen to the tissues 

 and cells where it is given up if needed. The latter is determined by the 

 amount of oxygen present in the tissue, the activity of the tissue, etc. The 

 carbon dioxide is removed from tissues by the plasma. 



Excretion. — Some of the wastes are excreted by the frog skin and in- 

 testine (Figs. 208 and 213), but many are taken from the blood by a pair 

 of elongated kidneys in the dorsal abdominal cavity. Internally, a kid- 

 ney contains a number of Malpighian bodies, each consisting of an en- 

 closing membrane known as Bowman s capsule, which surrounds a coiled 

 mass of thin- walled capillaries known as a glomerulus (glo -mer' u lus) 

 (L. glomus, ball). Wastes are collected from the blood in the glomeruli 

 and carried by urinijerous tubules to collecting tubules and thence to the 

 tubular ureter and finally to the saclike cloaca (klo -a' ka) (L. cloaca, 

 sewer) (Fig. 213). From the latter the urine may be stored in the thin- 

 walled, distensible urinary bladder, which voids only at certain intervals. 

 Ciliated, funnel-shaped nephrostomes in the ventral part of the kidney 

 open into the coelom, from which wastes may be secured and later 

 eliminated. 



Coordination and Sensory Equipment. — The nervous system may be 

 divided into ( 1 ) central nervous system, consisting of brain and spinal 

 cord, (2) peripheral nervous system, consisting of ten pairs of cranial 

 nerves and ten pairs of spinal nerves, and (3) sympathetic nervous sys- 

 tem, consisting of nerves and ganglia which supply the internal (visceral) 

 organs (Figs. 216 and 217) . 



The brain has the following structures: (1) two small, fused olfac- 

 tory lobes for the sense of smell, (2) two large, elongated cerebral hemi- 

 spheres of uncertain function, (3) two large optic lobes for the sense of 



