ATROPHY OF ORGANS FROM LACK OF USE 267 



(6) Atrophy of muscles. Cessation of use is also 

 the cause of the degeneration of the flexor and 

 extensor muscles of the fore-limb in Cetacea, and 

 of the imperfection of the finger joint articulations 

 in Cetacea and Sirenia (fig. 79). In the latter 

 cases the surfaces of the articu- 

 lation which make flexor and 



extensor movements easy dis- 

 appear. When a limb becomes 

 a paddle, it is necessary that it 

 should be flexible, but that the 

 articulations should be immobile. 



(7) Atrophy of the tail in 

 man. The caudal region of the 

 human vertical column is com- 

 posed of four or five very de- 

 generate vertebrae. The whole 

 of this organ is degenerate. 

 When the tail is formed at an 

 early stage of embryonic life, no. 



.-i ,L i i i . D melas. Right anterior 



the vertebral column consists of an sbowinl absence of 



.,.. i , ,1 , i i articular facets for the 



thirty-eight vertebrae ; the lesser joints of the angers. 

 number of vertebrae in the adult 

 is due to reduction of the tail, which in man is 

 quite useless. Later on in life a further instance 

 of atrophy may occur in individuals. In old men 

 the caudal vertebrae are frequently fused, and the 

 whole region is smaller. 



(8) Degeneration of the hyoid apparatus in man 

 and birds. This case shows a close correspon- 



