124 THE LATERAL VENTRICLES. 



grey matter is the nucleus caudatus of the corpus striatum (c. sir.} It is pear-shaped, with 

 the larger end projecting into the floor of the anterior horn, and the smaller tapering 

 posterior end (tail) extending to the origin of the descending horn, and then curving 

 downwards and forwards along the dorsal part of this horn, in which it forms a 

 long, narrow, rounded eminence. The posterior horn (figs. 88, 89, c.p.), which 

 is curved around the parie to- occipital fissure, its concavity therefore being 

 directed inwards, passes, as before said, into the occipital lobe. It is roofed by the 

 fibres of the corpus callosum which are passing to the temporal and occipital lobes ; 

 part of these fibres also form its lateral wall. At the upper part of its mesial wall 

 there is a bundle of fibres (forceps major) sweeping round from the splenium of the 

 callosum to enter the occipital lobe. This produces a projection into the cavity of 

 the horn, which is known as the lulb of the posterior horn. Below it is another 

 curved eminence, which extends from the base of the horn backwards for a variable 

 distance, to end in a pointed extremity : this is termed, from its resemblance to a 

 cock's spur, the ergot (Morand), or calcar avis (also hippocampus minor). It 

 corresponds with the calcarine fissure on the mesial surface of the hemisphere. 



The hippocampus minor is not peculiar to the human brain as was at one time thought, but 

 is found in the brains of quadrumana. In the human subject the posterior horn varies 

 greatly in size, and the hippocampus minor is still more variable in its development, being 

 sometimes scarcely to be recognised, and at others proportionally large. It is usually most 

 developed where the posterior horn is longest ; but the length of the posterior horn, and 

 prominence of the hippocampus minor, are by no means in proportion to the dimensions 

 of the hemisphere. 



The middle, lateral, or descending horn (fig. 89) is directed at first back- 

 wards and outwards from the posterior end of the body of the ventricle ; it then 

 passes downwards and forwards with a bold sweep (at about the plane of the parallel 

 sulcus), being finally curved inwards, and ending about an inch from the apex of 

 the temporal lobe, where it abuts against the amygdala. The dorso-lateral boundary 

 (roof) is formed by the fibres of the callosum, which are arching over it into the 

 temporal lobe, forming the so-called tupetum. The tail of the nucleus caudatus 

 and the taenia semicircularis are prolonged into the roof. Extending along the floor 

 of the horn is an eminence known as the hippocampus major or cornu Ammonis (fig. 

 89, ti), which becomes enlarged as it descends, and being notched, its edge here 

 presents a certain resemblance to an animal's paw (pes hippocampi). This eminence 

 corresponds with the hippocampal fissure externally, which thus indents the floor 

 of the cornu, and the grey matter at the bottom of the fissure being separated from 

 the cavity of the ventricle by a thin layer of white substance (covered with ependyma"), 

 shews through and gives a bluish-white appearance to the hippocampus. Mesial to 

 the hippocampus is the white band known as the fimbria (fig. 89,./z), prolonged from 

 the posterior pillar of the fornix ; and over the fimbria, at the angle which the floor 

 of the horn here forms with its roof, the choroid plexus projects into the cavity from 

 the external pia mater ; the plexus is, however, covered by the epithelial lining 

 of the cavity which extends over it in passing from roof to floor. If the pia mater 

 is pulled away it drags along with it this covering layer of epithelium, and the cornu 

 is made to open on the mesial surface of the brain. A cleft-like opening is thus 

 produced, which has been called the inferior fissure of the c&rebrum, being the 

 lower part of the so-called transverse fissure, which follows the plane of the velum 

 interpositum (see p. 184) over the thalamus and third ventricle, and emerges over 

 the corpora quadrigemina. It is along this fissure that the choroid plexuses of the 

 pia mater are invaginated into the ventricles, covered by the ventricular epithelium, 

 which is pushed in before them. It is only when the choroid plexuses are dragged 

 away that an actual fissure is formed in the hemisphere-wall between the fornix 

 and fimbria on the one side and the optic thalamus on the other ; but with the 



