FUNCTIONS OF THE CEREBRUM. 361 



the convolutions of the anterior and posterior Cerebral lobes. Above this, 

 again, is the superior longitudinal commissure, which is formed by the fibrous 

 matter of the great convolutions nearest the median plane on the upper sur- 

 face of the Cerebrum, and which connects the convolutions of the anterior and 

 middle lobes with those of the posterior. Beneath the Corpus Callosum, we 

 find the most extensive of all the longitudinal commissures, the Fornix. This 

 is connected in front with the Thalami optici, the Corpora mammillaria, the 

 tuber cinereum, &c. ; and behind it spreads its fibres over the hippocampi 

 (major and minor), which are nothing else than peculiar convolutions that 

 project into the posterior and descending cornua of the lateral ventricles. 

 The fourth longitudinal commissure is the Tsenia semicircularis, which 

 forms part of the same system of fibres with the fornix ; connecting the cor- 

 pus marnmillare and thalamus opticus of each side with the middle lobe of 

 the cerebral hemisphere. If, as Dr. Todd has remarked,* we could take 

 away the corpus callosum, the grey matter of the internal convolution, and 

 the ventricular prominence of the optic thalami, then all these commissures 

 would fall together, and would become united in the same series of longitu- 

 dinal fibres. Experiment does not throw any light upon the particular func- 

 tions of the Corpus Callosum and other Commissures; since they can 

 scarcely be divided without severe general injury. It would appear, how- 

 ever, that the partial or entire absence of these parts, reducing the Cerebrum 

 (in this respect at least) to the level of that of the Marsupial Quadruped, or of 

 the Bird, is by no means an unfrequent cause of deficient intellectual power. 



a. The following case of deficient commissures, lately recorded by Mr. Paget, is of much 

 interest. The middle portion of the Foraix, and the whole of the Septum Lucidum, were 

 absent; and in place of the Corpus Callosum, there was only a thin fasciculated layer of 

 fibrous matter, l - 4 inch in length, but of which the fibres extended to all the parts of the brain, 

 into which the fibres of the healthy corpus callosum can be traced. The Middle commissure 

 was very large ; and the lateral parts of the Fornix, with the rest of the Brain, were quite 

 healthy. The patient was a servant-girl, who died of pericarditis. She had displayed, during 

 her life, nothing very remarkable in her mental condition, beyond a peculiar want of forethought, 

 and power of judging of the probable event of things. Her memory was good ; and she possessed 

 as much ordinary knowledge as is commonly acquired by persons in her rank of life. She 

 was of good moral character, trustworthy, and fully competent to all the duties of her station, 

 though somewhat heedless ; her temper was good, and disposition cheerful. The mental de- 

 ficiencies in the few other cases of which the details have been recorded, seem to have been 

 of the same order; and this is exactly what might Irave been anticipated; since the depriva- 

 tion of these parts takes away that, which is most characteristic of the Cerebrum of Man and 

 of the higher Mammalia; and their intellectual operations are peculiarly distinguished by that 

 application of past experience to the prediction of the future, which constitutes the highest effort 

 of Intelligence. 



475. The weight of the entire Encephalon in the. adult Male usually ranges 

 between 46 and 53 ounces; and in the Female, from 41 to 47 ounces. The 

 maximum of the healthy brain seems to be about 64 ounces, m four pounds; 

 and the minimum about 31 oz., or something less than two pounds. But in 

 cases of idiocy, the amount is sometimes much below this; as low a weight 

 as 20 oz. having been recorded. It appears, from the recent investigations 

 of M. Bourgery, that the relative sizes of the different component elements 

 of the Human Encephalon are somewhat as follows. Dividing the whole 

 into 204 parts, the weight of the Cerebrum will be represented by about 170 

 of those parts, that of the Cerebellum by 21, and that of the Medulla Oblon- 

 gata with the Optic Thalami and Corpora Striata at 13. The weight of the 

 Spinal Cord would be, on the same scale, 7 parts. Hence the Cerebral He- 

 mispheres of Man include an amount of nervous matter, which is four times 



* Anatomy of the Brain, Spinal Cord, &c., p. 234. 

 31 



