704 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



influence of the special sense-organs which connect with the encephalon. The 

 physiological reactions of the higher vertebrates are especially modified by this 

 latter arrangement. It is therefore true that the cord, as well as the brain, is, 

 in man, more complicated anatomically than in any of the lower forms, and 

 this in spite of the fact that the independent reactions of the human cord are 

 so imperfect. 



One result of this concentration of the nerve-elements toward the head, 

 and the dependence of the rest of the system on the encephalon, is, as we shall 

 see, that the cephalic division becomes thereby a more necessary portion of the 

 pathway for the incoming impulses, and, conversely, as cephalization fails to 

 take place the several parts of the system remain more independent. 



Reactions of Portions of Spinal Cord. When an amphioxus is cut 

 into two pieces and then put back in the water, a slight dermal stimulus 

 causes in both of them locomotory movements, such as are made by the 

 entire animal. 



When a shark (Seyllium caniculd) is beheaded the torso swims in a co-ordi- 

 nated manner when returned to the water. Separation of the cord from the 

 brain does not deprive a ray (Torpedo oculata) of the power of perfect loco- 

 motion. The same is true of the ganoid fish. In the case of the cyclostome 

 fish (Petromyzon) the beheaded trunk is, in the water, inactive, and, on gentle 

 mechanical stimulation it makes inco-ordinated responses, but, put in a bath 

 formed by a 3 per cent, solution of picro-sulphuric acid, locomotion under the 

 influence of this strong and extensive dermal stimulus is completely performed. 

 In the case of the eel the responsiveness even to the picro-sulphuric acid bath 

 is evident in the caudal part of the body alone. In the bony fish this power 

 in the spinal cord has not been observed. 1 



In these experiments the central system is represented by the entire spinal 

 cord with the associated nerves, or by some fraction of it, but so simple, con- 

 stant, and independent are the reactions of the cord under normal conditions 

 that a strong stimulus is able to elicit the characteristic responses from even a 

 fragment of the system. The higher we ascend in the vertebrate series the 

 less evident do the independent powers of the cord become. 



For the determination of the functions of the several parts of the nervous 

 system it is possible to employ in animals the method of removal as well as 

 the method of stimulation. The doctrine of localization was at one time 

 crudely expressed by the statement that a cortical centre was one the stimula- 

 tion of which produced a given reaction, and the removal of which abolished 

 this same reaction. Goltz 2 soon showed that in the dog the removal of even 

 an entire hemisphere did not cause a paralysis of the muscles on the opposite 

 side of the body, although others had shown that a stimulation of certain por- 

 tions of the cortex of the hemisphere would cause these muscles to contract. 

 It was argued, therefore and quite rightly that the cortical centres of the 

 dog did not completely answer to the definition. 



1 Steiner : Die Functionen des Cenlrcdnervensystems und ihre Pkylogenese, 2te Abth., " Die 

 Fische," 1888. 2 Ueber die Verrichtungen des Grosshims, 1881. 



