STRUCTURE OF THE VERTEBRATES 219 



voice mechanism is only slightly changed in the reptiles. In the 

 mammals, however, with the development of the thyroid car- 

 tilages, the vocal cords stretch from the arytenoids to the thyroid 

 cartilages. This increased length, correlated with the greatly de- 

 veloped muscular and nervous control of the larynx, gives the 

 mammals a fuller control of pitch and tone. The principle of 

 voice is based on the fact that a taut membrane vibrates more 

 rapidly and with a higher pitch than one less tightly stretched. 

 Differences in normal voice pitch in the mammals are due to the 

 length of the membranes. Under equal tension a long cord will 

 vibrate more slowly than a short one. The deeper voice of the 

 male is caused by the growth of the larynx in a dorso-ventral 

 direction at puberty. Length of larynx and depth of voice in a 

 species are definitely correlated with the prominence of the 

 Adam's apple. 



The voice box of the birds is an entirely different structure, 

 being located at the bifurcation of the trachea. This syrinx is a 

 remarkably efficient organ as is demonstrated by parrots, mock- 

 ing birds, and other birds which have ability for mimicry and 

 a wide variation in pitch. 



C. Other Respiratory Structures 



In addition to gills and lungs, a number of animals have mod- 

 ifications of other structures which have a respiratory function. 

 The only one of wide distribution is the skin which serves as a 

 respiratory organ in most urodeles and many anura. In water 

 living urodeles the cutaneous artery along the side of the body 

 is larger than the pulmonary artery. Even the frog, when at rest 

 and in a moist location, can secure sufficient oxygen through the 

 skin. The most highly developed case of skin breathing is in some 

 salamanders which lack both gills and lungs in the adult condi- 

 tion, and carry on their entire respiration through the skin. 

 Cutaneous breathing is necessarily limited to those animals 

 which have a very thin, highly vascular skin, and live in wet 

 regions. For these reasons it is not found in any group above the 

 amphibia. 



An isolated, but interesting, modification is found in the South 

 American dipnoans. As was mentioned on page 46, the pelvic 



