746 OF THE FUNCTIONS OF THE NERVOUS SYSTEM. 



posterior. Beneath the Corpus Callosum, we find the most extensive of all the 

 longitudinal commissures, the Fornix. [Fig. 194. ED.] 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 Taenia semicircular is, which forms part of the same system of fibres with 

 the fornix ; connecting the corpus mammillare and thalamus opticus of each 

 side with the middle lobe of the cerebral hemisphere. If, as Dr. Todd has 

 remarked, 1 we could take away the corpus callosum, the gray 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 longitudinal fibres. Experiment does not throw any light upon the 

 particular functions 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. 2 



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

 between 40 and 60 oz., the average being about 50 oz. ; and in the Female, 

 from 36 to 50 oz., the average being about 45 oz. The maximum of the 

 healthy brain seems to be about 64 oz., and the minimum' about 31 oz. But in 

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

 20 ounces 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 Oblongata 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 Hemispheres of Man 



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



2 The following case of deficient commissures, recorded by Mr. Paget (" Medico-Chirurg. 

 Transactions," vol. xxiv.), is of much interest. The middle portion of the Fornix, 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-4th 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 some- 

 what heedless ; her temper was good, and disposition cheerful. The mental deficiencies 

 in most of 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 have been anticipated ; since the de- 

 privation of these parts takes away that which is most characteristic of the Cerebrum of 

 Man and of the higher Mammalia ; their intellectual operations being peculiarly distinguished 

 by that application of past experience to the prediction of the future, which constitutes one of the 

 highest efforts of Intelligence. Another case has been since put on record by Mr. Mitchell 

 Henry (Op. cit., vol. xxxi.), in which the anterior portion of the Corpus Callosum was defi- 

 cient, together with the middle and anterior portion of the Fornix, and the whole of the Sep- 

 tum Lucidum. There was in this case, also, a marked intellectual deficiency, but apparently 

 of a different character from that which showed itself in the preceding case ; for, instead 

 of vivacity and habitual rapidity of action, there was here a disproportionate degree of 

 slowness in action, amounting almost to stupidity. The difference in the two cases, how- 

 ever, is perhaps to be set down rather to the account of general temperament; since in 

 both of them there seems to have been a deficiency in the power of carrying on a continuous 

 train of thought. 



