250 PHYSIOLOGY FOR DENTAL STUDENTS. 



the cell of this neurone is located in the anterior horn of 

 matter of the cord. These anterior horn cells are distinguished 

 from the other nerve cells of the grey matter by their large si/c 

 and angular shape, and they become greatly increased in num- 

 ber in the portions of the cord from which the nerves going to 

 the extremities originate. The fibers springing from them pass 

 out in the anterior roots. If the cells are destroyed or the an- 

 terior roots cut, degeneration occurs below the lesion, and para- 

 lysis of the effector organs (muscles) to which they run results, 

 but this paralysis is very slight in degree unless the lesion af- 

 fects several roots, or the cells of several adjacent levels of the 

 cord. The reason for this is that the nerve cells of one level of 

 the cord only partially supply a given muscle or group of mus- 

 cles with nerve fibers, thus showing that even the small muscles 

 receive their nerve fibers from several adjacent levels of 

 the cord. The anterior horn cells sometimes become destroyed 

 by disease, namely, in infantile paralysis (poliomyelitis anter- 

 ior). The resulting paralysis is never recovered from. 



Types of Reflexes. Having traced the paths through which 

 reflexes occur in the higher animals, we may now proceed to 

 consider certain typical forms of reflex action and the condi- 

 tions which may cause them to become altered. We must first 

 of all confine our attention to the characteristic reflexes of the 

 so-called spinal animal, for it is only after we have done so that 

 it will be possible for us to determine what influence the brain 

 has in modifying the spinal reflexes. The spinal animal (dog, 

 for example) is prepared by cutting across the spinal cord some- 

 where below the origin of the phrenic nerves. After the imme- 

 diate effects of the operation have been recoverd from, the 

 regions of the animal's body, lying below the level of the sec- 

 tion of the cord, suffer from a condition called spinal shock. 

 All reflex movements are absent, the sphincters are paralyzed so 

 that incontinence of urine and faeces exists, and various "tro- 

 phic" or nutritive changes occur in the skin (abscesses form, 

 hair falls out, etc.). After some time, the length of which de- 

 pends on the position of the animal in the animal scale, the 

 sphincters regain their tone and the reflexes gradually reappear 



