136 PHYSIOLOGY. 



is supposed to be the point at which excitement to motion 

 commences and sensation terminates, and to possess the 

 power of originating motion in itselfj Mr. Mayo infers 

 this, from the fact that the brain proper, the optic tubercles, 

 and even the cerebellum may be removed by successive por- 

 tions, and still the animal live, and exhibit feeling and in- 

 stinct ; but if the medulla oblongata be removed, all con- 

 sciousness is immediately destroyed. Other physiologists 

 say, that it is the hemispheres of the cerebrum that take 

 cognizance of sensation, and the source from whence volition 

 proceeds ; and that though they may be removed, layer 

 after layer, without pain, yet by so doing, the senses are 

 destroyed, memory and intelligence abolished, and the animal 

 reduced to an automaton, destitute of design, apprehension, 

 and judgment. As all the nerves which supply the larynx, 

 lungs, heart, stomach, and external muscles of respiration 

 originate from it, its removal must instantly destroy life. 



4. (The spinal cord passess down through the foramen 

 magnum, or great hole of the skull, and the canal of the ver- 

 tebra, till it reaches the second vertebra of the loins, where 

 it generally terminates in a tapering pointj Besides the 

 bony case in which it is lodged, the cord is enclosed by a 

 prolongation of the dura mater, which, however, does not 

 adhere immediately to the bone, as it does in the skull. It 

 also is supplied with extensions of the arachnoid and pia 

 mater ; between which, there is usually a quantity of serous 

 fluid, varying from two to six ounces, which seems to impart 

 a certain degree of support by its pressure and obviates the 

 effects of concussion J 



5. The general form of the spinal cord is cylindrical, 

 though somewhat flattened, from its transverse diameter 

 being the greatest. Its thickness, however, is not uniform, 

 as it bulges out at the lower part of the neck, where the 

 nerves go off to the arms, and also at the lower part of the 

 back, from whence spring the nerves of the legsj. It is com- 

 posed of grey and white, or cineritious and medullary mat- 



