THE NERVOUS SYSTEM. 441 



It is not to be supposed that the nervous communications between 

 the successive deposits of gray matter are necessarily of so simple a 

 character as those represented in Fig. 154. This is only a diagram, 

 representing the general fact of the longitudinal connection existing 



Fig. 154. 



DIAGRAMATIC SECTION OP HUMAN BRA i N ; showing the situation of the nervous 

 centres and the longitudinal tracts of white substance. 1, Olfactory lobe. 2, 2. Convolutions 

 of the cerebral hemispheres. 3. Corpus striatum. 4. Optic thalamus. 6. Tubercula quadri- 

 gemina. 6. Crura cerebri. 7. Tuber annulare. 8. Cerebellum. 9. Medulla oblongata. 



between the spinal cord and the different parts of the encephalon. It 

 is by no means certain that all or any of the fibres of the cord run con- 

 tinuously through the medulla oblongata, the tuber annulare, and the 

 cerebral ganglia, to the gray matter of the convolutions. On the con- 

 trary, careful examination of successive microscopic sections by the best 

 observers have failed to show such a direct continuity. It appears more 

 probable that the fibres coming from the spinal cord terminate in the 

 medulla oblongata, and that other fibres originating from the gray matter 

 of the medulla pass upward, partly to the cerebellum and partly to the 

 corpora striata and optic thalami ; while other fibres still, originating 

 from these ganglia, diverge thence to form the connection between them 

 and the cerebral convolutions. According to this view, the longitudinal 

 tracts of white substance consist of nerve fibres which are interrupted 

 in their course by the nerve cells of different deposits of gray matter, 

 so that an impression or impulse conveyed from one to the other is not 

 the same throughout its course, but is modified by the action of the 

 nervous centres which successively receive and transmit it. 



Each portion of the cerebro-spinal axis has its right and left halves 

 connected with each other by transverse commissures, and sends out 

 nerves, containing motor and sensitive fibres, to corresponding regions 

 of the body. The spinal cord supplies the integument and muscles of 

 the neck, trunk, and extremities. The medulla oblongata sends out 

 motor and sensitive nerves to the muscles of the head and face, and to 

 the skin and mucous membranes of the same region; while it also sup- 

 29 



