554 



THE NERVOUS SYSTEM 



Usually, impulses are transmitted to a nerve cell only over its dendrite, 

 but it must be also assumed that such a conveyance of impulses may take 



place over the collaterals of its axone near the 

 cell body, or the cell body may be stimulated 

 directly by the afferent neurone. The per- 

 ipheral fiber of the spinal ganglion cell., 

 although it has the structure of an axone, may 

 be looked upon physiologically as a dendrite, 

 since homologues in lower vertebrates and 

 in man himself (nerve cells of the ganglion 

 of the cochlea of the auditory nerve) have 

 this structure, the nerve cell body being 

 situated near the sensory surface from which 

 impressions are received. 



Irradiation of Impulses within the 

 Cord. Taking as an example a frog whose 

 brain has been destroyed, a simple reflex may 

 be demonstrated by irritating the skin of one 

 foot with a weak stimulus. In response to 

 such a stimulus the foot is flexed upon the leg, 

 due to a contraction of the muscles of the 

 reflex arc corresponding to the sensory surface 

 irritated. If the strength or duration of the 

 stimulus be increased, other groups of muscles 

 are involved 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 action in a frog 

 which has its brain destroyed. 



The fact is that in the development of the 



FIG. 369. Scheme of Lower 

 Motor Neurone. The cell body, 

 protoplasmic processes, axone, 

 collaterals, and terminal arbor- 

 izations in muscle are all seen to 

 be parts of a single cell and 

 together constitute the neurone. 

 (Barker.) c, Cytoplasm of cell 

 body containing chromophilic 

 bodies, neurofibrils, and peri- 

 fibrillar substance; n, nucleus; n', 

 nucleolus; d, dendrites; ah, axone 

 hill free from chromophilic bodies; 

 ax, axone; sf, side fibril (col- 

 lateral); m, medullary sheath; nR, 

 node of Ranvier where side 

 branch is given off; si, neuri- 

 lemma and incisures of Schmidt; 

 m, striated muscle fiber; tel, 

 motor end plate. 



nervous system certain physiological paths of slight resistance have been 

 established between the sensory areas and the muscles which move the parts 



