Focal Symijtoms. 



595 



sened sensibility of the paralyzed portion of the body. In ani- 

 mals this partial loss of sensibility is, as a rule, difficult or im- 

 possible to recognize on account of the disturbance of conscious- 

 ness which usually accompany such diseases. A symptom that 

 is far more easily observed is the anesthesia or hyperesthesia 

 of the portions of the skin or the mucous membranes supplied 

 by the vagus or the trigeminus in cases where either the nucleus 

 or the root process of either of these nerves is diseased. 



In diseased conditions in- 

 volving an interruption of 

 the optic tract there are al- 

 so disturbances of vision 

 (see Fig. 83). 



It has been shown by Dexler that 

 in the horse the greater part of the 

 fibers of the optic nerves decussate 

 and that the proportion of the fibers 

 from the inner portion of the retina 

 (nasal portion) which decussate to 

 those from the outer or temporal por- 

 tion which do not cross is 5:1. Thus 

 it happens that the optic tract and the 

 more central portion of the nerve path 

 of vision contain fibers coming from 

 both eyes and the greater number of 

 these are derived from the inner por- 

 tion of the eye of the opposite side 

 (fig. 83). The optic tract is similar 

 to the above in the ruminants and 

 swine, whereas in carnivora the pro- 

 portion of the fibers which cross to 

 those which do not is smaller. 



Interruption of the optic nerve 

 causes amaurosis of the eye, dilatation 

 of the pupil and pupillary reaction is 

 lost. Regarding the lesions induced 

 in the chiasma, optic tract, thalamus, 

 corpus geniculare laterale, optic radia- 

 tion of Gratiolet, and center of vision 

 by destruction of the nerve path noth- 

 ing definite is known. Hemianopia 

 has been observed by Probst in dogs 

 resulting from lesions in the region of 

 the thalamus produced experimentally, 

 and by E. Balint as a result of lesions 

 in the occipital cortex. A similar con- 

 dition is seen in the human subject in 

 cases of unilateral injury to the optic 

 path posterior to the chiasma. Hemi- 

 anopia is an insusceptibility to light 

 in the temporal portion of the retina 



on the same side as the lesion, and in the nasal region on the opposite side. Total 

 destruction of the chiasma would lead to blindness in both eyes and complete absence 

 of pupillary reaction, but owing to the difference in the proportion of fibers which 

 decussate in carnivora and herbivora lesions of the path posterior to the decussation 

 determine hemianopia in the former, whereas in the latter the disturbance of one 

 eye is far greater than that of the other. In view of the fact that in animals inves- 

 tigations of the power of vision cannot be so complete as in man the disturbances of 

 vision mentioned pro^sably pass unnoticed save in carnivora. Besnoit found total 

 blindness on the left side in a cow affected with tuberculosis and softening due to 

 thrombosis of the right hemisphere. 



In cases where there is disease of the optic path between the chiasma and the 



Fig. 83. Diagram of the optic paths in 

 the horse. R. Retina of the left eye, dark 

 portion showing the uncrossed fibers and 

 light portion showing the crossed fibers. 

 No. Optic nerve. Tro. Commencement of 

 tlie tract behind the chiasma. CM. Mey- 

 nert's commissure. CG. Gudden's commis- 

 sure. Tho. Optic thalamus. Cgl. Lateral 

 corpus geniculare. Qa. Anterior corpora 

 quadrigemina. SG. Optic radiation of 

 Gratiolet. Co. Occipital cortex (optic cen- 

 ter). (Adapted from Dexler and Ober- 

 steiner. ) 



