THE CAT. 



[CHAP. ix. 



only in its isolation, but in its nature. It is in no way a prolonga- 

 tion forwards of the spinal canalis centralis. 



This fornix is made up of two white cords closely approximated 

 anteriorly and diverging widely behind. Each springs from the 

 corpus albicans, and the two cords (called pillars or crura) ascend 

 (side by side) behind the anterior commissure, and with a branch of 

 the foramen of Monro on the outer side of each. They then curve 

 backwards, diverging, but at the same time united by that delicate 

 membrane called the lyra. They become connected with the corpus 

 callosum, and then pass into each descending cornu of the lateral 

 ventricles. 



Two rounded bodies (the optic thalami) are placed one on each 

 side of the first described cavity (the third ventricle), and are con- 

 nected by the soft (or middle) and posterior commissures. Two 

 other rounded bodies (the corpora striata) are placed one in each 

 cerebral hemisphere between the anterior and descending cornua. 

 They are connected by the anterior commissure, which (like the 

 posterior commissure) is formed of white, transverse fibres, while the 

 soft commissure is almost entirely composed of grey matter. 



Each corpus striatum is an outgrowth from the middle of the base 

 of its hemisphere, and is the morphological axis of the whole 

 hemisphere. 



The optic thalami are thickenings in the outer wall of the third 

 ventricle. 



The two cords which have been spoken of as the optic tracts, 

 arise, one on each side, from the optic thalami and run forwards 

 obliquely across the under surface of the brain to join together 

 immediately in front of the infundibulum. 



We have seen that the two sides of the brain are connected by 

 the fibres of the corpus callosum and of the three smaller commis- 

 sures. There is not only this direct transverse connection. Oblique 

 extensions of fibres also connect the right hemisphere with the left 

 side of the spinal cord (and therefore with the left side of the body), 

 and the left hemisphere with the right half of the spinal cord (and 

 therefore with the right side of the body). The fibres which pass 

 forwards through the " anterior pyramids " decussate and then 

 extend through the crura cerebri to the cerebrum. The fibres of 

 the crura radiate within the cerebrum in a fan-like manner, the 

 corpus striatum and thalarnus being respectively anterior and 

 posterior to such radiation. 



The transverse relation which thus exists between the two sides 

 of the brain and the body (which it supplies with nerves) extends 

 also to the organs of sight, but not to that of smell. 



We may now, in conclusion, review the conditions presented by 

 the cerebral convolutions, i.e., by the gyri and sulci of the cerebrum. 

 These are thus disposed as follows : 



On the upper surface of the brain three more or less parallel 

 prominences or gyri, extend antero-posteriorly on each side of the 

 median longitudinal fissure. The innermost of these, the superior 



