126 PHILOSOPHY OF ZOOLOGY. 
occasioned partly by the air in the ear, and partly by the 
motion of the bloodvessels *. 
The changes which take place in the tenacity of muscles 
after death, are very remarkable. The same force which 
they could resist with ease, in a living state, is sufficient to 
tear them to pieces after the vital principle has departed. 
The tendons, which are sometimes lacerated by the violent 
contractions of the muscle durmg life, are much stronger 
than the muscle itself after death. These circumstances 
serve to confirm us in the belief of a vital power. If it 
does not exist, it remains with the mechanical and chemical 
philosophers to determine, what strengthening principle has 
departed with life, and whence the weakness of death. 
Muscles, from their mode of action, may be divided into 
circular and longitudinal. In the circular muscles, the 
fibres are so arranged, that they contract the part to which 
they are applied in all its dimensions, as the muscles of the 
heart and tongue; or they merely contract the extremity 
of a tube, so as to close it. In this last case, they are de- 
nominated sphincters. ‘The longitudinal muscles may be 
compared to ropes, drawing towards them the objects to 
which they are attached in a direction influenced. by the 
form of these objects; and the other muscles acting in con- 
cert. Where the one extremity of the muscle is attached 
to a hard part, and the other to a soft, the mechanism is 
usually of the simplest kind. But where both extremities 
are fixed to hard parts, which are destined to be used as 
levers, more complicated machinery is requisite. We have 
already adverted to the different forms of the joimts of the 
bones suited to different movements. The muscles are 
fixed to these bones at various angles, and various distan- 

* Phil. Trans. 1810, p. 2. 
