212 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



the amount of central nervous matter which must be present in the frog's 

 spinal cord in order that the reactions can take place. 



Reactions from Fractions of the Cord. — -If the construction of the cord 

 was strictly segmental (a condition nearly approached in some worms and 

 arthropods), in the sense thai each segment contained the associated nerves 

 for a given band of skin and muscle, there should be no disturbance on 

 dividing the cord into its anatomical segments; and practically among the 

 invertebrates, where the ganglionic chain is thus arranged, the single segments 

 can perform alone all the reactions of* which they are capable under normal 

 conditions. 1 In such invertebrates the only change effected by the combina- 

 tion of the segments is that of co-ordinating in time and in intensity the 

 reactions of the series. If, on the other hand, the segments of the cord were 

 more or less dependent upon one another, and not physiologically equivalent, 

 modifications of various degrees would arise according to the segments isolated. 

 It has been found that the spinal cord of the frog may, under special condi- 

 tions, be reduced to three segments and reactions still be obtained. 



During the breeding season the male frog, by means of his fore legs, clasps 

 the female vigorously and often for days. If, at this season, there is cut out 

 from the male the region of the shoulder girdle bearing the fore limbs together 

 with the connected skin and muscles, and the three upper segments of the 

 spinal cord, then an irritation of the skin will cause a reflex clasping move- 

 ment similar to that characteristic for the normal male at this season. 



Reactions in Other Vertebrates. — It must not be thought, how r ever, 

 because it is the custom to emphasize the reflex activities of the lower verte- 

 brates, and to show that these reflexes can be carried out even by fractions 

 of the spinal cord alone, that, therefore, the spinal cord is particularly well 

 developed in them. Comparative anatomy shows in the lower vertebrates a 

 simplicity in the structure of the cord quite comparable with that found in 

 the brain, and, as we ascend the vertebrate series, both parts of the central 

 system increase in complexity. In this increase, however, the cephalic divi- 

 sion takes the lead ; and further, by means of the fibre-tracts from it to the 

 cord, the cell-groups in the cord are more and more brought under the influence 

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

 logical reactions of the higher vertebrates are especially modified by this 

 hitter 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 is true in spite of the fact that the independent reactions of the human 

 cord are so imperfect. 



When an amphioxus is cut into two pieces and then put back in the water, 

 a slight dermal stimulus cause- in both of them locomotor movements, such 

 as are made by the entire animal. 



When a shark (Scyllium canicula) is beheaded, the torso swims in a co-or- 

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



1 Loeb: Einleilung in dit vergleichen.de Gehirnphysiologie und vergleichende Psychologie, Leip- 

 zig, 1899. 



