THE CEREBRUM. 791 



membrane secretes a limpid and transparent fluid, analogous to the cerebro- 

 spinal fluid, though in health it is always in small quantity. 



3. The Septum Lucidum (Fig. 429, 24). 



This appellation is given to a thin middle band, standing vertically between 

 the two lateral ventricles, elongated from before to behind, widened considerably 

 at its anterior extremity, terminating in a point at its posterior extremity, and 

 inserted above into the corpus callosum, below into the back of the fornix. 



On the faces of this partition, which is formed of white substance, is spread 

 the pro]3er membrane of the lateral ventricles. In the human species, a narrow 

 ventricle has been described as found in its substance ; but this does not appear 

 to exist in the domesticated animals. 



4. The Fornix, or Trigonum (Fig. 435, 3), 



Also named the vault of three or four pillars, the fornix (arch) is a single 

 middle body in the interior of the brain, concurring to separate the two ven- 

 tricles, and serving to support the septum lucidum. It is depressed below and 

 above, and is of a triangular form ; its apex, looking downward, stands in the 

 median plane above the foramen of Monro and the thalami optici, though sepa- 

 rated from the latter by the velum interpositum and the hippocampi, and 

 receives on its upper face the insertion of the septum lucidum. Behind, at its 

 base, and in the middle line, the fornix is confounded with the corpus callosum, 

 which it supports ; it is prolonged on each side by a lamina extending to the 

 surface of the hippocampus, forming the cortical layer of this deep convolution 

 of the brain, and with its congener constituting the posterior pillars (posterior 

 crura, tcenice itippocampi, or corpora fmbriata) of the fornix. These two pillars 

 are united by some white, transverse fibres, which form what has been named 

 the lyre. 



In front, at its apex, the fornix is also attached to the corpus callosum, and 

 divides into two cords or anterior pillars (crura) — (Figs. 425 ; 429, 17), which 

 pass in front of the anterior cerebral commissure, are inflected downwards and 

 backwards, in traversing the optic thalamus, on the sides of the middle ventricle, 

 and finally have their extremities confounded with the mammillary process 

 (corpus albicans). 



These two crura limit, in front, the foramen commune anterius, ov foramen of 

 Monro (Fig. 425, 16), over which the apex of the fornix is thrown across like an 

 arch (Fig. 425). 



The fornix is white throughout its whole extent, with a greyish tint towards 

 its summit. 



5. The Hippocampi (Fig. 435, 4). 



The hippocampus, or cornu Ammonis (from its resemblance to a ram's horn, 

 the crest of Jupiter Ammon), is an elongated projection, a veritable internal 

 convolution of the brain (is, in fact, the internal surface of the gijrus fornicatus, 

 or convolution lying upon the corpus callosum, and which terminates at the 

 fissure of Sylvius). It occupies the floor of the anterior part of the lateral 

 ventricle, and is prolonged throughout its reflected portion, the curvature of 

 which it exactly follows. Considered together, the two hippocampi somewhat 

 closely resemble the uterine cornua of the Cow. 



By their internal extremity, they are in contact with each other beneath the 

 middle portion of the fornix, and above the optic thalamus, which is separated from 



