MUSCULAR SYSTEM. 135 
which lift lips, nose and lids and those which move the ears—muscles 
which as a whole have their highest development in man (fig. 142). 
THE DIAPHRAGM. 
The diaphragm is a transverse voluntary muscle which crosses the 
body cavity of the mammals just behind the pericardium and lungs. 
Its muscles are in part derived from those of the back, in part from the 
rectus muscles of the lower surface. Various attempts have been made 
to recognize similar muscles in the lower vertebrates, in some cases 
with considerable success. Its development is outlined in the section 
on the ccelom (p. 123). The diaphragm is dome-shaped and is attached 
to the vertebral column and to the ribs. It is traversed by the 
cesophagus and the large arterial and venous trunks. In some verte- 
brates the muscular portion is confined to the margin, the centre being 
membranous; in others the muscle fibres extend across it. Contrac- 
tion of the muscles flatten it, thus enlarging the pleural cavities and 
drawing air into the lungs, thus aidingin respiration. Itis supplied by 
the phrenic nerve. 
ELECTRICAL ORGANS. 
It is well known that the contraction of a muscle causes the dis- 
charge of a minute amount of electrical energy, so it is not surprising 
that in certain cases muscles are modified into electrical organs. ‘The 
known cases occur only in elasmobranchs and teleosts. The discharge 
is weak in most species, but is strong in Torpedo and Gymnotus. In all 
but Malapterurus the electrical organs are clearly modified muscles, situ- 
ated in the head in Torpedo and Astroscopus, in the trunk of Gymnotus, 
and in the tail of Raia, the nerve supply being correspondingly varied. 
Thus in Torpedo the seventh, ninth and tenth cranial nerves are con- 
cerned, while in Gymnotus and the skates the supply is from the spinal 
nerves. Malapterurus is peculiar in that the organ is in the integu- 
ment and has been supposed by some to arise from modified glands. 
It is more probable that here as elsewhere it is derived from the muscles, 
as the organ is under control of the will; the development has yet to be 
studied. This diversity of origin clearly shows that the electrical 
functions have been separately acquired in the different species. 
The organs are composed of a large number of electrical plates 
