THE NERVOUS SYSTEM 



in the following order: i, Those of the leg and thigh of the same side; 2, 

 homologous muscles of the opposite side; 3, the arms of the same side and of 

 the opposite side. 



The increasing complexity of the reflexes aroused by stimulation of one and 

 the same sensory spot is not easy of explanation. We know that there is almost 



an infinite number of morphological paths in 

 the cord, yet the responses are orderly and 

 observe a certain sequence in their increasing 

 complexity. The reflexes have a mechanical 

 definiteness which, in a living structure, seems 

 almost purposeful, yet there is no conscious- 

 ness in a frog which has its brain destroyed. 



The fact is that in the development of the 

 nervous system certain physiological paths of 

 slight resistance have been established between 

 the sensory areas and the muscles which move 

 the parts for their protection. Apparently 

 other physiological nerve pathways exist, but 

 it requires a stronger sensory stimulus to 

 arouse nerve impulses along these paths. In 

 explanation we may suppose that the stronger 

 afferent impulses are sufficient to overcome 

 the resistance of increasingly complex paths, 

 that they diffuse through greater and greater 

 extents of the cord. But we may repeat that 

 in the normal state of the cord this diffusion is 

 in an orderly physiological sequence. 



Orderly reflexes can be called forth only 

 by stimulating sensory nerve endings, the 

 first of the essential structures of the reflex arc. 

 If artificial stimuli are applied to a nerve 

 trunk, as the sciatic, uncoordinated muscular 

 responses occur because the sensory stimuli 

 are diffuse and general and are not specific 

 and local. 



An involvement of multiple pathways may 

 also be accomplished through decreasing the 

 resistance within the cord, as through the use 

 of some drug such as strychnine. In the 

 strychninized frog a slight stimulus brings 

 about multiple and violent reflex spasms. 



These contractions have lost their orderliness and are uncoordinated. The 

 entire musculature contracts. It is as though the strychnine removed all 



FIG. 367. Scheme of Lower Mo- 

 tor Neurone. The cell body, proto- 

 plasmic processes, axone, collaterals, 

 and terminal arborizations in muscle 

 are all seen to be parts of a single cell 

 and together constitute the neurone. 

 (Barker.) c, Cytoplasm of cell body 

 containing chrompphilic bodies, neu- 

 ro fibrils, and perifibrillar substance; 

 n, nucleus; n', nucleolus; d, den- 

 drites; ah, axone hill free from chro- 

 mophilic bodies; ax, axone; sf, side 

 fibril (collateral); m, medullary 

 sheath; nR, node of Ranvier where 

 side branch is given off; si, neu- 

 rilemma and incisures of Schmidt; 

 m, striated muscle fiber; tel, motor 

 end plate. 



