Anatomy of the Brain. 563 



the middle cerebral arter}^ There is usually in adults no well marked 

 separation between the middle and posterior lobes. In the foetus, how- 

 ever, there is a distinct furrow between them. In the horse, ox, and 

 other mammals, the posterior lobe is rudimentary or wanting ; the mid- 

 dle, or mastoid lobe extending back far enough to merely cover the an- 

 terior bulbs of the corpora quadrigemina, the nates, while the cerebel- 

 lum covers the testes. But in man and the apes the posterior lobes ex- 

 tend to the rear, so as to cover not only the corpora quadrigemina but 

 the cerebellum itself. 



The two hemispheres are connected together by a great band of trans- 

 verse white fibres, called the corpus callosum. This band extends far 

 enough back to be above the corpora quadrigemina, and far enough for- 

 ward to cover the corpora striata. The fibres pass under the falx cere- 

 bri in crossing from side to side, and turn up into the body of each 

 hemisphere, mingle with the fibres which constitute the central parts of 

 the hemispheres, and, without doubt, connect with the gray cells which 

 make up the cortex. On top of the corpus callosum, in the middle, 

 are two longitudinal bands of white medullary matter running from the 

 front to the hind end. They are called the chordae longitudinalis. The 

 corpus callosum is the great commissure of the hemispheres. 



At its anterior extremity the corpus callosum bends downward form- 

 ing a knee or genu. The posterior end also bends under, the fibres be- 

 coming mixed up with those of the body of the fornix. This bend is 

 called the splenium. The fibres from the different parts of the corpus 

 callosum have been given different names. Those which leave the an- 

 terior end and curve forwards and inwards to accommodate the anterior 

 lobe are the Forceps anterior. Those which leave the posterior end and 

 curve backwards to supply the posterior lobe are the Forceps. While 

 those which pass directly outward from the same into the middle lobes 

 are the Tapetum. 



Underneath the corpus callosum are the cavities called the lateral ven- 

 tricles. These cavities are close together in front, where they are sepa- 

 rated by the partition called the septum lucidum, but they diverge from 

 each other toward the rear. In the bottom of the front ends of the 

 cavities lie the corpora striata. The end of each cavity is continued 

 forward to a point called the anterior cornu or horn. In nearly all the 

 vertebrates there is a canal opening from the extremity of the anterior 

 cornu into the olfactory lobe. In man this is closed. The backward 

 projection of the lateral ventricle is outward from the center, then one 

 prong passes downward toward the base of the brain with a final turn to 

 the front ending in the substance of the brain by a blind sac or point. 

 This is called the descending or middle cornu in man and the apes, and 

 its location is in the second or middle lobe. In the higher mammals, 



