32 NERVOUS SYSTEM. 



and the physiological experiments of Legallois lead us now to 

 consider the spinal cord as the principal nerve-centre of the 

 organism. Experimental researches have been principally 

 concerned with the spinal cord, and the characteristics dis- 

 covered here have been, reasoning by analogy, extended to 

 other portions of the nervous system. 



Nerve Centre, Gray Matter, Nerve Commissures. In the 

 actual state of our knowledge, the three principal objects in 

 the central nerve masses are : the brain, the spinal cord, and 

 the small nerve centres called ganglia (system of the grand 

 sympathetic) distributed through the visceral cavities ; these 

 latter have slight, if any, connection with the brain. But the 

 exact notions that we possess are applied almost exclusively 

 to one of these objects; viz., the spinal cord and its ence- 

 phalic portion (bulb, protuberance). 



at, 



Fig. 11. Transverse section of the spinal cord in man.* 



From an anatomical point of view the central portions are 

 characterized by the presence of nerve-cells; but from a 

 physiological point of view they are characterized by the 

 reflex act. 



Nerve globules of the spinal cord form in this organ a 

 central continuous mass (gray substance, gray axis) stretch- 

 ing from one end of the organ to the other. (Fig. 11.) But 



* Cervical region (10 diameters), f^ Posterior columns, n, Gelatinous sub- 

 stance of the posterior horn. &, Posterior root. ?, Anterior roots, a, Anterior 

 median fissure, c, Posterior median fissure, b, Central canal of tne cord, g, An- 

 terior horus. h, Posterior horns, e, Autero-lateral column. 



