Human Physiology. 



175 



this creamy stuff, which is 95-iooths pure water, and the rest 

 tubes, membrane, cells. 



The nerves, which seem to be but extensions of the brain- 

 nerve matter carried into the remotest parts of the body, are 

 soft white tubes, as shown in Fig. 29, 

 rilled with cellular matter (3), and ter- 

 minating in loops, both in the muscles, 

 as already shown, and in the organs 

 of sensation. 



Press on a nerve or divide it and 

 its function ceases, but a divided nerve 

 may re*-unite. It is sometimes neces- 

 sary in surgical operations for neuralgia 

 to cut out a piece of the nerve. A 

 knock on the head will produce un- 

 consciousness and general paralysis of the voluntary muscles. 

 The heart continues to beat, the lungs expand, all the in- 

 voluntary, vital functions go on, for they have a nervous system 

 of their own in the body the nervous system of organic life, 

 or ganglionic system. The brain, though the seat of conscious- 

 ness, thought, and volition, is not necessary to the life of the 

 body. The bodies of infants are sometimes formed without 

 brains. A turtle lives for days with its head cut off. The 

 system is built up, and all its unconscious interior work of 

 nutrition, circulation, secretion, excretion carried on by the 



Fig. 29. MINUTE NER- 

 VOUS STRUCTURE.* 



* Fig. 29 represents the microscopic elements of the nervous structure. 

 i. Mode of termination of white nerve-fibres in loops ; three of these loops 

 are simple, the fourth is convoluted. The latter is found in situations 

 where a high degree of sensation exists. 2. A white nerve-fibre from the 

 brain, showing the varicose or knotty appearance produced by traction or 

 pressure. 3. A white nerve-fibre enlarged to show its structure, a tubular 

 envelope and a contained substance. 4. A nerve-cell, showing its com- 

 position of a granular-looking capsule and granular contents. 5. Its nucleus 

 containing a nucleolus. 6 A nerve-cell, from which several processes are 

 off ; it contains also a nucleated nucleus. 7. Nerve-granules. 



