88 
MR. OWEN ON THE STRUCTURE OF 
laminae of the septum lucidum by which the fornix is connected with the corpus cal- 
losum, present an extent of surface corresponding to the degree to which the corpus 
callosum and fornix recede vertically from one another as they advance from behind 
forwards. In tracing the modifications of these different parts through the mam mi - 
ferous series, the disproportion of the fornix to the corpus callosum is found to de- 
crease as the parts, to the connexion of which they are subservient, alter in their re- 
lative size. For as the superincumbent masses of the cerebral hemispheres diminish 
in the placental Mammalia, the corpus callosum is proportionally restricted in its 
development ; while the hippocampi and their free processes, called the taenise hip- 
pocampi, maintaining a remarkable uniformity in their absolute size, the fornix also 
continues large, and undergoes modifications of form which more distinctly manifest 
its relation as a commissure to the hippocampi than its structure in the human brain 
would indicate. Thus in the brain of the Sheep the taeniae hippocampi, instead of 
being lost in the posterior crura of the fornix, are continued along its lateral margins, 
augmenting its breadth : they converge and unite above the anterior crura of the 
fornix, which here appear as small subordinate appendages sent off into the optic 
thalaini below, from the union of the taeniae above ; the taeniae are then again sepa- 
rated and are continued downwards and forwards into the anterior lobes of the hemi- 
spheres, bringing these parts into communication with the hippocampi behind, whilst 
the point of union of the opposite taeniae becomes continuous with the anterior fold of 
the corpus callosum above. 
As the corpus callosum and fornix recede vertically from one another in a less de- 
gree in most Mammalia than in Man, the two laminae of the septum lucidum are 
consequently of less extent, but are proportionally stronger ; they are formed not 
merely by the epithelium of the lateral ventricles, but by fibrous laminae extending 
from the anterior and upper surface of the fornix to the opposite surface of the corpus 
callosum. In the simple and depressed forms of brain, such as the Rodentia present, 
the fornix, or hippocampal commissure, and the corpus callosum, or hemispheric com- 
missure, are in contact, so that their uniting medium cannot with propriety be termed 
the septum lucidum. 
The corpus callosum is the principal bond of union between the opposite hemi- 
spheres, extending horizontally above the ventricles, its middle fibres passing trans- 
versely, while those of its extremities, which are more or less bent beneath its body, 
radiate, and all intermix, in apposition with the ascending and diverging fibres of 
the peduncles of the cerebral hemispheres. It has hitherto been considered as the 
great characteristic of the brain in the Mammalia, and, taking the human brain as 
the term of comparison, to be developed in the ratio of the magnitude of the cerebral 
hemispheres. 
In the placental Mammalia this is a pretty accurate expression of the relations of 
the corpus callosum ; and as the posterior lobes of the hemispheres are the first to 
disappear in the descending comparison, so the corpus callosum diminishes in longi- 
