THE MEMBRANES OF THE BRAIN. 1197 



of Letters and of Art have possessed a weight little above, or sometimes even below, the aver- 

 age. In this connection it must be remembered that it is not improbable that the cortical cells 

 of different brains vary in their capacity for activity and in their power of retaining impressions; 

 that, in short, differences of quality exist. Further, that notwithstanding the possible low gen- 

 eral weight of a brain, the amount of the cortical gray matter, especially of certain regions con- 

 cerned in some particular phase of mental activity, may exist in unusual abundance. Moreover, 

 it is probable, from the investigations of Kaes l , that actual increase of the functioning associa- 

 tion fibres takes place in response to the stimulus induced by excessive exercise of certain parts 

 of the cortex. It is evident, therefore, that as applied to the individual, brain-weight alone 

 affords little dependable information as to intellectual power, and that brains which, judged 

 from their weight, apparently have been ordinary, may have been exceptional in the amount of 

 cortical gray matter and, perhaps, in the unusual capacity of their neurones. 



Considered, however, in relation to great groups, as to peoples or to races, brain-weight has 

 been found to correspond to the general plane of intelligence and culture. In this connection 

 the observations of Bean 2 are suggestive. He found the average brain-weight of the male negro 

 to be 1292 gm., with extremes of 1010 gm. and 1560 gm. ; that of the male Caucasian 1341 gm., 

 with extremes of 1040 gm. and 1555 gm. Notwithstanding the relatively low class of the 

 white subjects examined, the average weight of their brains was greater than that of the 

 high-class negroes. Bean concludes that the smaller size of the negro brain is primarily in the 

 frontal lobe, and, therefore, that the anterior association centre is relatively and absolutely smaller. 



The observations of E. A. Spitzka 3 concerning the area of the corpus callosum in median 

 sagittal section, call attention to the unusual size of this commissure in the brains of men of con- 

 spicuous intellectual power. Moreover, in the particular group of brains thus examined varia- 

 tions in the details of the callosa strikingly suggested well-known differences in the mental traits 

 of the persons during life. The validity of the area of the callosum as a trustworthy index as to 

 intellectual capacity has been seriously affected by the fact, illustrated by Retzius and by Bean, 

 that callosa of uncommon size usually belong to brains of high weight, and that not infrequently 

 such brains are from individuals of ordinary or even of low intelligence, as exemplified by the 

 cases of Bean, among which a number of callosa of very large area were from low-class-whites 

 and even from negroes. 



THE MEMBRANES OF THE BRAIN. 



Like the spinal cord, the brain is enveloped by three membranes, or meninges, 

 which, from without inward, are : (i) the dura mater, (2) the arachnoid and (3) 

 the pia mater. The first of these is closely applied to the inner surface of the cra- 

 nium, of which it constitutes the periosteum, and, in addition, by means of its processes 

 serves to support and guard from undue pressure the enclosed mass of nervous tissue. 

 The pia mater is the vascular tunic carrying the blood-vessels for the nutrition of the 

 brain and, therefore, lies in contact with all parts of the external surface of the organ ; 

 whilst the arachnoid, the thinnest and most delicate of the three coats, is free from 

 blood-vessels but is intimately related with the intracranial lymph-paths. Although 

 the dura and the pia are closely attached to the skull and the brain respectively, they 

 are separated by an interval which, in turn, is subdivided into two compartments by the 

 arachnoid. The outer of these clefts lies between the dura and the arachnoid and is 

 called the subdural space ; the other, between the arachnoid and the pia, is the 

 subarachnoid space. The first of these spaces is usually a mere capillary cleft, the 

 arachnoid lying against the dura, and contains a small amount of a clear light straw- 

 colored fluid of the nature of lymph. The second one, although much more capa- 

 cious than the subdural, is crossed by so many trabeculae of arachnoid tissue that 

 in many places it acquires the character of a sponge-like tissue, rather than of an 

 unbroken channel. Whilst anatomically the subdural and the subarachnoid spaces 

 are distinct and nowhere communicate, as demonstrated by careful artificial injections 

 into the subdural cleft, it is probable that during life the cerebro-spinal fluid finds its 

 way through the thin partition of arachnoid tissue and enters the subdural space. 

 The interstices of the arachnoid are filled with the cerebro-spinal fluid, a modified 

 lymph, which is produced by the choroid plexuses within the ventricles. After dis- 

 tending these cavities, the fluid gains the subarachnoid space by way of the foramen 

 of Magendie and the foramina of Luschka situated in the attenuated roof of the fourth 



1 Die Grosshirnrinde des Menschen, 1907 



2 Amer. Journal of Anat., vol. v., 1906. 



3 Amer. Journal of Anat., vol. iv., 1905. 



