69 1 THE CENTRAL AXIS OF THE NERVOUS SYSTEM. 



This disposition permits the division of the cerebral ventricles into two 

 regions : an anterior, and a posterior or reflected. 



The anterior region is separated in the median plane from the opposite 

 ventricle, by the septum lucidtim and the summit of the fornix, beneath 

 which is the foramen of Monro establishing a communication between the 

 middle and the two lateral ventricles, and between these latter. Above, it 

 offers a smooth wall formed by the corpus callosum. Below, on its floor, 

 there is first remarked, in front, the corpus striatum ; behind, the internal 

 portion of the hippocampus ; in the middle, an oblique groove running from 

 behind to before, and without inwards, at the bottom of which floats the 

 choroid plexus. The anterior extremity of this region, occupied by the 

 base of the corpus striatum, is continued by a narrow opening into the 

 interior of the olfactory lobe. The posterior is prolonged, without any line 

 of demarcation, by the reflected portion of the ventricular cavity. 



The latter region occupies the most declivitous portion of the posterior 

 lobe of the hemisphere, and presents a strongly curved canal whose con- 

 cavity looks forward ; this canal terminates in a cul-de-sac in the substance 

 of the mastoid lobule. On the floor of this canal is delineated the posterior 

 portion of the hippocampus and the choroid plexus. 



A very fine membrane the ventricular arachnoid plays the part of a 

 serous membrane and covers the walls of these cavities, being spread every- 

 where over a layer of white substance, prolonged into the ethmoidal diverti- 

 culum, and continuous, through the foramen of Monro, with that of the middle 

 ventricle. This membrane secretes a limpid and transparent liquid, analogous 

 to the cerebro-spinal fluid, though in health it is always in small quantity. 



3. The Septum Lucidum. (Fig. 327, 24.) 



This appellation is given to a thin median lamella, 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 lamella, which is formed of white substance, is 

 spread the proper membrane of the lateral ventricles. In the human species, 

 a narrow ventricular cavity has been described as found in its substance ; 

 but this does not appear to exist in our domesticated animals. 



4. The Fornix (or Trigonum). (Fig. 313, 3.) 



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

 and median body in the interior of the brain, concurring to separate the two 

 ventricles, and serving to support the septum lucidum. It is depressed from 

 below to 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 separated from the latter by the velum interpositum and the hippo- 

 campi, and receives on its upper face the insertion of the septum lucidum. 

 Behind, towards its base, and on the median plane, 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, or corpora flmbriata) of the 

 fornix. 



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



