THE NERVES OF MOTION AND OF SENSATION, AND THE BRAIN. 259 



When the two tracts or columns which descend from the posterior portions of the 

 crura cerebri are transversely divided, where they form the slit of the calamus scrip- 

 torius, and when they are dissected down (fig-. 4. a, a, b.), we obtain a very interesting 

 view of the back part of the anterior columns (fig. 4. d, d.), or rather of the pyramidal 

 bodies, and their decussation. We see the union and decussation of these bodies before 

 they separate and descend to form the motor columns of the spinal marrow. The 

 motor and sensitive columns, — which were close together in tJie crura cerebri, and 

 which in their descent were separated in the pons, and by the septum which is con- 

 tinued down from the posterior transverse septum of the pons, — come here again into 

 contact at the point of union and decussation*. The motor columns approach the 

 sensitive columns, but no union takes place; the columns keep their respective 

 courses down the spinal marrow. When we dissect these parts carefully at the 

 back of the medulla oblongata, we may feel, and with sharp eyes we may see, very 

 minute and yet uncommonly strong filaments which run among these parts. We 

 may consider such filaments as a further proof how carefully these textures are 

 g-uarded against laceration. 



When the dissection is carefully made, we have thus a view of the posterior part 

 of the decussation of the pyramidal bodies ; and after their decussation we see them 

 separate and descend in the two anterior or motor columns. 



Concluding view of the Sensitive and Motor System of Nerves. 



If it could be said hitherto that the distribution of the nervous system, more than 

 any other part of the animal structure, evinces design, the conclusion is irresistible, 

 when we perceive that the parts which minister to sensation and motion are arranged 

 with a symmetry beyond what we expect to see in architectural plans or ornaments, 

 where every part is balanced, and each has its counterpart. 



It could not be well imagined that sensation and motion belonged to parts sepa- 

 rate and dissimilar. Formerly I believed that the nerves of sensation, that is to say, 

 the posterior roots of the spinal nerves, came from the posterior columns of the spinal 

 marrow, and consequently from the cerebellum. Whilst entertaining this belief I 

 found my progress barred, for it appeared to me incomprehensible that motion 

 could result from an organ like the cerebrum, and sensation from the cerebellum, 

 for there was no agreement between them. They conformed neither in size, shape, 

 nor subdivisions. Sensation and volition are necessarily combined in every action 

 of the frame -f. Although these influences, of whatever nature they be, are pro- 



in regular pellicles, we may else be deceived. On separating, for example, the posterior and lateral columns 

 at the true sulcus of separation, we shall see the minute transverse fibres : which appearance is produced by the 

 splitting of the septum. See the former paper. 



* The motor and sensitive columns do not mix or decussate, but only the motor columns with each other, 

 and the sensitive columns with each other. 



t This has been illustrated in former papers, and particularly in treating of the actions of the lips. 



