MEASUREMENTS OF THE BRAIN. 



1195 



About the fourth month, the last-named structure, which until this time is of uniform width, 

 exhibits a local thickening in its upper part just in front of the foramen of Monro and in 

 advance of the front-end of the choroid fissure. This thickening of the lamina terminalis, 

 at first oval in section, soon becomes pear-shaped with the point directed downward (Fig. 

 1032). The point enlarges and, after its later invasion by an ingrowth of transverse fibres, 

 forms the anterior commissure. The upper part of the thickened area expands in the 

 sagittal direction and is traversed by fibres which pass from one hemisphere to the other. 

 It thus becomes the corpus callosum. This structure soon assumes an elongated and 

 slightly arched form, but does not appropriate the entire enlarged upper part of the origi- 

 nally pyriform area. The antero-inferior portion, covered above by the corpus callosum, 

 remains thin and is converted into the septum lucidum. The fibres of the fornix appear early along 

 the choroidal margin of the hippocampus, forming a bundle of increasing size as it extends forward 

 over the foramen of Monro. The development of the corpus callosum is closely connected with 

 that of the fornix commissure (Streeter). The septum lucidum is at first solid although thin ; 

 subsequently it is partially separated into two lamellae by a narrow cleft, the so-called fifth ven- 

 tricle, which is completely closed, is devoid of an ependymal lining, and, therefore, is no part of 



Choroidal fissure 

 Primary gyrus dentatus 



Roof-plate of III ventricle 

 (taenia thalami) 



Thalamus 

 Posterior commissure 



Quadrigeminal plate 



Mid-brain cavity 



Cerebral peduncle 



Cerebellum 



IV ventricle 



Fig. 1032. 



Corpus callosum 

 Rostrum 



Septum lucidum 



Anterior commissure 



III ventricle 

 Olfactory lobe 



Lamina terminalis 

 Optic chiasm 

 Pituitary body 



Medulla 



Mesial surface of left half of human foetus of fourth month. X 2. {Marchand.) 



the system of true ventricular spaces. Concerning the manner and reason of its formation 

 opinions differ. The older view, that the space represents an isolated portion of the longitudi- 

 nal fissure cut off during the development of the corpus callosum, is sustained neither by its 

 history nor by the adult condition of the septum lucidum in many animals in which the partition 

 is solid and no space exits. Goldstein,^ however, accepts this view, while Marchand,^ His and. 

 others, regard the splitting as secondary. In consequence of the growth, increasing bulk and 

 backward extension of the corpus callosum and the fusion of the fornix along its under surface, 

 the primary upper part of the hippocampus, which extends well forward along the mesial surface 

 of the hemisphere, entirely disappears, its furrow, the hippocampal fissure, being later repre- 

 sented by the callosal sulcus, whilst the corresponding portion of the gyrus dentatus is reduced 

 to the atrophic sheet of gray matter and the longitudinal striae found upon the upper surface of 

 the corpus callosum. 



MEASUREMENTS OF THE BRAIN. 



The brain fits within the cranial case so accurately that its form is modified by 

 the general shape of the skull, being relatively long and ellipsoidal in dolichocephalic 

 subjects and shorter and more spherical in brachycephalic ones. The usual length 

 of the brain, measured from the frontal to the occipital pole, is from 160-170 mm. 

 (6^ in.) in male subjects and from 150-160 mm. (6 in.) in female. Its greatest 

 transverse diameter is about 140 mm. (5J4 in.) for both sexes and its greatest verti- 



' Archiv f. Anatom. u. Entwickelung., 1903. 

 ''Archivf. mikros. Anatom., Bd. xxxvii., i8qi. 



