n6o 



HUMAN ANATOMY. 



of the fornix. The septum consists of two thin layers (laminae septi pellucidi), 

 between which lies a narrow cleft (cavum septi pellucidi) to which the misleading 

 name, fifth ventricle, has long been applied. This space, very variable in extent 

 and width, is usually so narrow and contains such a small quantity of modified 

 lymph, that the laminae forming its walls are in apposition. It is entirely closed and, 

 therefore, cut off from the true ventricular system ; neither is it lined with ependyma. 

 The septum lucidum in man is the rudimentary representation of what in many 

 of the lower (macrosmatic) animals is a much more important tract of cortical 

 substance. In some animals, as for example, the rabbit, cat and dog, the septum 

 is solid, a cleft never appearing within it. Notwithstanding the reduction which 

 it has suffered in man, the septum exhibits in its structure its relation to the 

 cortex, comprising, from its cleft outward : (i) a thin layer of nerve-fibres, (2) an 

 uncertain layer of gray matter containing numerous nerve-cells of pyramidal form, 

 and, next to the lateral ventricle, (3) a layer of nerve-fibres, the ventricular surface 



of which is clothed with 

 FIG. 999. 



Body of fornix 



Corpus callosmn. 

 upper surface 



/ Anterior 



pillar of 



fornix 



Mammil- 

 la ry body 



the usual ependyma. It 

 is probable that axones 

 proceeding from the cells 

 within the septum lucidum 

 are constituents of the 

 olfactory strands within 

 the fornix, which pass to 

 the hippocampus and the 

 uncus, and of the tenia 

 semicircularis (page 

 1162), terminating in 

 the amygdaloid nucleus 

 (page 1172). 



The Lateral Ven- 

 tricles. The lateral 

 ventricles (ventricula late- 

 rales) are a pair of irreg- 

 ular cavities contained 

 within the cerebral hemi- 

 spheres. They are devel- 

 oped as outpouchings 

 from the original cavity 

 of the end-brain and for 

 a time communicate with this space by wide openings. The latter, however, fail to 

 keep pace in their growth with the expansion of the hemispheres, and in the fully 

 developed brain are represented by the small apertures of the foramen of Monro, 

 which maintains communication between the lateral and third ventricles, the last- 

 named space representing the primary, cavity of the fore-brain. 



When viewed from above, after removal of its roof, the corpus callosum and its 

 lateral extensions, each lateral ventricle appears as an elongated, irregularly curved 

 cavity (Fig. 1000), which extends for about two-thirds of the entire length of the 

 hemisphere and, in addition, penetrates the temporal lobe almost to its pole. It 

 is lined, as are all the other true ventricles, with a delicate epithelial layer, the 

 '/>< intv nut, which likewise clothes tin- structures which encroach upon its lumen, as 

 the caudate nucleus and the thalamus, as well as those which seemingly hang free 

 within it, as the choroid plexus and the fornix. It is usual to describe the ventricle 

 nsisting of four parts, the body, and the anterior, posterior and inferior horus. 

 The anterior horn and the body are practically one and separated by only an arbi- 

 trary division ; the posterior and the inferior horn extend into the occipital and the 

 temporal lobe respectively, whilst the anterior horn enters the frontal lobe. 



The anterior horn (cornu antcrius ) includes from the tip of the ventricle to 

 the foramen of Moiiro, the latter corresponding with the anterior limit of the con- 

 spicuous choroid plexus, curves forward and outward around the head of the caudate 

 nucleus into the white substance of the frontal lobe and in frontal sections (Fig. 



rneus, partially 

 cut away 



Dissection showing fornix in front and above ; drawn from preparation and 

 Sieger model. 



