692 NERVOUS SYSTEM. 



stance of the corpora striata. The fibres from the middle portion converge regularly to 

 the middle region of the external portions of the optic thalami. The fibres from the pos- 

 terior portion pass from behind forward and distribute themselves in the posterior portion 

 of the optic thalami. The fibres from the convolutions of the hippocampi and the fascia 

 dentata are lost in the gray substance lining the internal borders of the optic thalami. In 

 addition to these converging fibres and the curved commissural fibres connecting the 

 different convolutions of each hemisphere with each other, are commissural fibres which 

 connect the two hemispheres, as well as fibres connecting together the corpora striata 

 and the optic thalami of the two sides. 



Certain of the fibres converging from the gray substance of the hemispheres to the 

 corpora striata and optic thalami are probably connected with the cells in the gray mat- 

 ter of these parts. Other fibres pass through the corpora striata and optic thalami to 

 become finally connected with the fibres of the medulla oblongata, and, through the 

 medulla oblongata, with the columns of the spinal cord. Following the antero-lateral 

 columns of the cord from below upward, they ascend to the medulla oblongata, decussate 

 in the median line, and pass from the medulla to the brain. Certain of these ascending 

 fibres, which are nearly all continuations of the antero-lateral columns of the cord, ascend 

 to the brain by passing deeply through the pons Varolii ; other fibres ascend in the 

 cerebral peduncles, or crura cerebri ; and other fibres pass to the tubercula quadrigemina. 

 As the bundles of fibres ascend from the medulla oblongata, they become more and more 

 numerous by reinforcements of fibres, probably derived from the cells of the collections 

 of gray matter in their course. 



We have attempted, in the above sketch of the fibres of the brain, to give a succinct 

 account of the points that are most interesting from their physiological relations, and to 

 confine our description, as far as possible, to anatomical facts that have been definitively 

 settled and are now generally accepted. But, as we have betore remarked, the course 

 of the fibres and their connections are so exceedingly intricate, that we cannot rely 

 entirely upon purely anatomical investigations. The results obtained by anatomists should 

 be controlled, as far as possible, by physiological and pathological observations. When 

 anatomical researches are directly opposed to the conclusions to be deduced from experi- 

 ments upon living animals, in view of the great uncertainty of the former, it will generally 

 be reasonable to assume that they are erroneous or incomplete. We know, as the results 

 of experiments upon animals, that the motor stimulus is conducted from the brain by the 

 antero-lateral columns of the cord, and that the conducting fibres decussate at the medulla 

 oblongata. This fact has been verified by pathological observations, chiefly in cases of 

 injury to the brain-substance from haemorrhage, softening, etc. We know that impres- 

 sions are appreciated as sensations in some part of the cerebrum, and that the sensory 

 conductors also decussate ; as is shown by occasional paralysis of both motion and sensation 

 as a consequence of brain-lesions. It is evident, therefore, that sensory conductors pass to 

 the brain, but their precise course is not easy to determine. We have seen, in treating of 

 the action of the cord as a conductor, that sensory impressions are transmitted by the gray 

 substance alone, and it is probably through connections between the cells of the different 

 centres that these impressions are finally carried to the brain. The physiological fact of 

 the conduction of sensory impressions is fully confirmed by pathology, but its mechanism 

 has been very little if at all elucidated by anatomical researches. 



We have remaining certain anatomical points relating to the cerebrum, cerebellum, 

 tuber annulare, and medulla oblongata, to be described separately in connection with 

 these divisions of the encephalon. 



TJie Cerebrum. 



The anatomical description which we have just given of the encephalon will answer 

 for most of the points of physiological interest connected with the cerebrum. As we 

 have seen, the cerebrum constitutes more than four-fifths of the encephalic mass. Its 



