THE DIENCEPHALON OR 'TWEENBRAIN 617 



Reflex stimulation of an animal in this condition produces certain coor- 

 dinated movements which are unmistakably related to the movements of loco- 

 motion. For example, the left anterior and the right posterior extremities 

 become flexed, while the right anterior and the left posterior are extended 

 at the same time, and vice versa. Not infrequently these reflexes alternate, 

 beginning regularly with flexion of the extremity directly stimulated. At 

 the same time the head and neck are twisted toward the stimulated side; the 

 mouth is opened, the lips and the tongue are retracted, the eyelids opened, the 

 pupil is dilated ; the animal utters cries or groans, etc. These reactions, which 

 when the cerebrum is intact usually accompany painful sensations, sometimes 

 appear singly, sometimes in certain combinations. 



Rigidity of the triceps muscle can be intermitted by stimulation of the white 

 matter between the anterior and the lateral columns or by stimulation of cer- 

 tain peripheral nerves of the posterior extremity. The effect is felt chiefly on 

 the homolateral side, but to a less extent on the heterolateral side also. 



5. THE DIENCEPHALON OR 'TWEENBRAIN 



The many connections of the 'tweenbrain with the gray matter of the 

 cerebrum on the one hand and with the afferent nerve tracts on the other 

 speak in most eloquent terms for the great physiological importance of this 

 division of the brain. In fact, all the tracts in which we should expect to 

 find prolongations of the posterior root fibers (the main part of the fillet 

 layer, the superior peduncle of the cerebellum, the longitudinal bundle of 

 the formatio reticularis), and the fibers of the optic tract all enter the 

 'tweenbrain, from which in turn they are continued to the cerebral cortex. 

 The latter also sends out fibers to the 'tweenbrain, from which further efferent 

 tracts are given off (Flechsig). 



It is possible that the external geniculate body is the point of origin of the 

 efferent optic fibers, and that the reflexes discharged by optic stimuli are here 

 carried over to them. 



The experimental and clinical observations on the 'tweenbrain are not of 

 such a kind as to give us even a crude notion of its actual physiological pur- 

 pose in the normal brain. We can only say that it appears from clinical 

 observations and from the anatomical facts that the different nuclei in this 

 portion of the brain have different functions, and that as a result we have 

 here a fairly sharp localization of different paths and their connections. 

 Hence when the optic thalamus contains a sharply circumscribed lesion, cer- 

 tain afferent impulses are wanting, and for this reason, as v. Monakow ob- 

 serves, many complicated movements are deficient ; many others are abnormally 

 performed, certain components being overstimulated, certain others inhibited. 



On the other hand the conditions appear to be very favorable for substitu- 

 tion of functions in the optic thalamus, so that when lesions are not too 

 extensive the effects are only temporary or may be entirely wanting. This 

 is probably to be explained in part by a bilateral influence of the thalami 

 in which the commissura mollis connecting them together assumes a certain 

 significance. 



