640 FUNCTIONS OF TUB CEREBRO -SPINAL NERVOUS SYSTEM. 



being prevented and the sensory impression continuing, the impression is 

 radiated to the motor ganglia of the opposite side, and these muscles are 

 accordingly brought into play. We have here exhibited the results of a 

 mechanical disposition of parts, and not the operation of a conscious intel- 

 ligence. It must not hence be inferred, however, that there is any essential 

 difference in the endowments of the Spinal Cord, between Man and the 

 lower animals; or that any psychical agency exists in the latter case which 

 is wanting in the former. We have already seen that the existence of even 

 the most perfectly-adapted combination of different muscular actions, all 

 obviously bearing upon a definite object, cannot in itself justify our attribut- 

 ing this combination to design or voluntary choice on the part of the organ- 

 ism that executes it ( 448) ; whilst, on the other hand, to remove these 

 movements in any case from the category of automatic actions, would be to 

 assign to the Spinal Cord a power of consciously selecting and directing 

 them, such as we have every reason for believing to be limited to the higher 

 parts of the Cerebro-spinal centres. Now the very uniformity of the move- 

 ments in question, or the slight variation in effect which has been observed 

 to follow the application of different stimuli, is itself an indication that they 

 do not proceed from any purposive choice, but depend upon the special en- 

 dowments of those centres of reflex action, whence the impulses that call 

 them forth immediately issue to the nerves ; and hence the more marked 

 adaptiveness of the reflex actions performed by many of the lower tribes of 

 animals, can only be held to indicate that a larger share of such adaptation 

 is effected in them by what may be termed the mechanism of their nervous 

 centres, and that less is left to voluntary choice and direction, which can 

 only be safely trusted where a considerable amount of intelligence exists to 

 guide it ; a conclusion which accords well with what has been already 

 stated respecting the structural differences that seem to exist between the 



Spinal Cord of Man and that of the 

 inferior Vertebrata ( 486). If we 

 endeavor to combine the results ob- 

 tained in pathological cases and those 

 observed after experimental sections 

 of the Spinal Cord with the evidence 

 derived from microscopical investiga- 

 tion, it would seem probable that the 

 sensory or centripetal nervous fibres 

 terminate in the ganglionic cells of the 

 posterior horns of the gray substance 

 of the cord (Fig. 240). From these 

 processes are given off, one of which 

 ascends on the opposite side to the sen- 

 sory ganglia at the bnse of the Brain, 

 whilst others terminate in cells that 

 are in direct connection with those 

 lying in the anterior horns of the gray 

 substance. Generally speaking, sen- 

 sory impressions follow the former 

 and simplest path, and any move- 

 ments that may occur arc directed by 

 and subordinate to the will ; but if 

 these ascending fibres arc divided, 

 then the purely reflex action of the 

 Spinal Cord is exhibited, and the im- 

 pression is transmitted to one cell or 



s, Sensory root.. in, Motor root. 



The ordinary course of a sensory impression to 

 the orain is through tlie ^an^'lion cell, n, and fibre, 

 ', which crosses to the opposite half of the cord. 

 In Die event of this heinj,' divided, the impression 

 it transmitted through an intermediate ^aii^'Hon 

 cell, /;, to k'roup of motor cells in the anterior cornii, 

 <, and is the nee transmitted through a motor nerve, 

 M, to the mus< I' s of Die same side. If the' impres- 

 sion lie still slron^er, the muscles of tlie opposite 

 side are called into retlex action through the com- 

 ninnicatini,' til ire,,/. The group of motor cells may 

 also he culled into action hy the voluntary nerve, 

 /, which decussates at the lower part of the Medulla 

 Oblongata. 



