188 PHYSIOLOGY OF CENTRAL NERVOUS SYSTEM. 



nections of the neurons.. As shown in Figs. 84 and 85, the number of 

 processes, particularly the dendritic processes, is much greater in 

 the cortical cells of the higher animals; or, to put this fact in another 

 way, the number of cells in the cortex of the higher animals is much 

 less for an area of the same size than in lower animals. The amount 

 of in-between substance or the richness of the network of processes 

 is increased. This anatomical fact would indicate that the greater 

 mental activity in the higher animals is dependent, in part, upon the 

 richer interconnection of the nerve cells, or, expressed physiologic- 

 ally, our mental processes are characterized by their more numer- 

 ous and complex associations. A visual or auditory stimulus that, 

 in the frog, for instance, may call forth a comparatively simple 

 motor response, may in man, on account of the numerous associa- 

 tions with the memory records of past experiences, lead to psychi- 

 cal and motor responses of a much more intricate and indirect 

 character. 



The Phenomenon of Conditioned Reflexes. — The usual reflexes 

 effected through the spinal cord and the lower centers of the brain 

 are of a fixed nature. Light thrown on the retina causes always a 

 constriction of the pupil; a touch applied to the cornea causes a 

 winking movement; acid in the mouth is followed by a secretion of 

 saliva. These and similar reflexes are essentially invariable 

 responses to appropriate stimuH, and the mechanism involved, the 

 reflex arc, is an inherited structure already elaborated for use. 

 Pawlow designates such acts as unconditioned reflexes. In contrast 

 to them he describes a class of reflexes obtained through the cortex 

 of the cerebrum which he calls conditioned reflexes, because they 

 may be elaborated under certain conditions. They may be 

 brought into existence by a process of training or education, and 

 they are subject to various forms of inhibition, internal and external. 

 He has made a special study of these reflexes in connection with 

 the salivary glands. If a certain sensory stimulus is given, for 

 example, a note of 800 vibrations per second, it has no effect on 

 the flow of saliva. But if repeatedly the salivary gland is stimu- 

 lated by acid or food simultaneously with the sounding of the 

 note a conditioned reflex will be established, and the note, when 

 given alone, will cause a flow of saliva. When this conditioned 

 reflex is first established it may be obtained also with neighboring 

 tones, but with training the reflex becomes more and more special- 

 ized, so that finally as small an interval as one-eighth of a tone 

 may be differentiated — a sound of 800 vibrations causes a flow 

 of saliva while a sound of 812 vibrations is ineffective. The 

 process of differentiation is explained as being due to an internal 

 inhibition, as shown by the following experiment. If a tone of 812 



