660 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



has developed the idea of the summation of stimuli, implying at some part of 

 the pathway a piling up of the effects of the separately inefficient stimuli to 

 a point at which they ultimately become effective. 



The details of the changes involved in this summation and the place at 

 which the changes occur are both obscure, but it would seem most probable 

 that summation is an expression of changes in the relations between the final 

 twigs of the afferent elements and the cell-bodies of the central or efferent 

 elements, which permit the better passage of the impulse from one element to 

 the other, for the evidence strongly indicates that the course of the impulse can 

 be interrupted at these junctions. The foregoing paragraphs are concerned, 

 therefore, with changes occurring in the afferent portion of the pathway. 



Next to be considered is 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 in the sense that 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. In such invertebrates the only change effected by the combination 

 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, modifica- 

 tions of various degrees would arise according to the segments isolated. It has 

 been found that the spinal cord of the frog may under special conditions 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. 1 



The Efferent Impulses. Incessantly the efferent impulses pass out from 

 the cord to the muscles and glands. With each fresh afferent impulse those 

 which go out are modified in strength and in their order, but just how they 

 shall be co-ordinated is dependent on so many and such delicate conditions that 

 even in the simplest case the results are to be predicted only in a general way. 



The attempt to determine the spread of the impulse in the cord by deter- 

 mining the order in which the various muscles of the thigh and leg contracted 

 in response to thermal stimuli was made by Lombard. 2 In a reflex frog the 

 tendons of the leg and thigh muscles were exposed at the knee, and each 

 attached to a writing rod in so ingenious a manner that simultaneous records 

 of fifteen muscles could sometimes be obtained. The stimulus was a metal 



1 Goltz : Centralblatt fur die medicinische Wissenschaften, 1865. 



2 Archiv fur Anatomic und Physiologic, 1885. 



