THB LOWER VERTEBRATA. 141 



sacculus rotundus, but passes abruptly into a much wider 

 tube, the large intestine. There is no caecum at the junction 

 of the two. The large intestine is short, and cannot be 

 divided into colon and rectum. Its final portion, however, 

 receives the openings of the urinary and genital ducts on its 

 dorsal side and that of the urinary bladder ventrally : this 

 portion is called the cloaca. 



4. Respiration. The glottis opens into the larynx (or 

 laryngo-tracheal chamber), which is a small chamber ventral 

 to the throat. From this the lungs come off immediately 

 in the absence of a neck there is neither room nor need for 

 a trachea. The lungs are simple sacs, their internal surface 

 somewhat increased by ingrowths from the walls, but 

 infinitely simple in comparison with the highly complex 

 lungs of the rabbit. The mechanism of respiration, too, is 

 quite different, as must be evident from the absence_of_hoth 

 diaphragm- ftBd=^ribs. In their stead we find the hyoid 

 relatively much larger than the rabbit's, being a large plate 

 of cartilage in the floor of the mouth. Muscles (sterno- 

 hyoid) run from this to the shoulder-girdle, and when they 

 contract pull the hyoid backwards and ventralwards, and so 

 enlarge the mouth cavity. Another set (petrohyoid) runs 

 from the hyoid upwards to the skull, and when these con- 

 tract they lift the hyoid and so diminish the capacity of the 

 mouth cavity. These two sets of muscles (in combination 

 with minor ones) act in respiration. First the nares are 

 opened, the floor of the mouth lowered ; air, of course, rushes 

 into the cavity. Then the nares are closed, the glottis opened, 

 and the floor of the mouth raised. The air is, of course, 

 forced into the lungs. Expiration takes place by the elastic 

 recoil of the lungs and the viscera that press on them, as 

 soon as the nares are re-opened. Thus the mechanism of 

 respiration in the frog may be called a buccal force-pump, 

 while that of the rabbit is a thoracic suction-pump. 



The lungs are not the sole respiratory organs. There is, 

 as we shall see, an abundant supply of deoxygenated blood 

 to the skin, and here also some interchange of gases between 

 air and blood can take place. This is said to be the only 

 means of respiration during hibernation, in winter. 



