96 NERVOUS TISSUE. 



depend less upon the antagonism of alkali and acid than on that of 

 the solid muscular fibrils (syntonin) and the muscular juice. 



Although we purpose at a future page reverting to the inter- 

 esting observations of the younger Liebig, we cannot abstain from 

 making a few remarks in relation to them in the present place. 

 It would appear from these experiments that muscle is dependent 

 on oxygen to enable it to contract ; for Liebig found that frogs' 

 muscles retained their contractility much longer in an atmosphere 

 of oxygen than in air which did not contain oxygen, as, for instance, 

 in carbonic acid, nitrogen, or hydrogen. It was further shown by 

 two carefully conducted experiments that, whilst the muscle is in 

 a state of contraction, oxygen is absorbed, and a corresponding 

 quantity of carbonic acid is exhaled, facts which confirm the pre- 

 viously advanced proposition, that a large portion of the carbonic 

 acid formed in the animal body is generated not in the capillaries, 

 but in the parenchyma of the organs, and is especially produced 

 by muscular action. We need scarcely remark that the results of 

 these observations are of the highest importance in relation to the 

 theory of metamorphosis of matter, whilst they afford powerful 

 support to the purely physical hypothesis of the elder Liebig in 

 relation to this subject. 



NERVES AND BRAIN. 



THE nervous system and the brain contain nerve-fibres and 

 nerve-cells as their special and peculiar morphological constituents. 



The nerve-fibres or tubes which are distributed throughout the 

 peripheral nerves, the spinal cord, the brain, and the ganglia, do 

 not all present the same appearance, when seen under the micro- 

 scope ; hence the nerve-fibres have been divided into two kinds, 

 namely, the thicker (animal, cerebro-spinal), and the more delicate 

 (sympathetic, vegetative, organic) ; there is no ramification obser- 

 vable in either of these classes of fibres, except at their extreme 

 terminations. 



The thicker nerve-fibres form cylindrical filaments, varying 

 from 0*004 to O'OIO'" in diameter; they occur especially in the 

 nerves springing from the spinal cord, but they are also met with 

 in other parts of the nervous system. When these nerve-fibres 

 are examined under the microscope in a perfectly fresh state, 

 especially after the addition of a little albumen, they appear 



