NERVOUS SYSTEM 843 



tions of the side-wall thus form a little island in the depths of the longi- 

 tudinal fissure. This island is called the insula (of Reil). 



The bending itself of the downward and forward growing parts has 

 caused a deep transverse fissure in each hemisphere known as the lateral 

 cerebral fissure or fissure of Sylvius. 



All higher forms of mammals have the brain substance thrown into 

 many folds or convolutions known as gyri (Fig. 473). 



The deeper grooves separating the gyri are called fissures, while 

 the lesser grooves are known as sulci. This folding permits a great 

 amount of cortex or gray matter to be provided for ; for, it will be noted 

 that not only do the tops of each convolution form cortex, but also the 

 entire sides of every sulcus. 



The hemispheres are divided into various lobes : frontal, parietal, 

 temporal and occipital. The two hemispheres are connected by various 

 commissures, which likewise must be studied in the actual brain and 

 compared with the diagram. 



Following are the chief commissures (Fig. 471) : 



Anterior commissure, in all vertebrates. 



Pallial commissure, dorsal to the anterior. This appears in verte- 

 brates from the amphibians upward. 



Corpus callosum (Fig. 471), and 



Hippocampal commissure. These last two are a variation in the 

 higher mammals of the pallial commissures in the lower. The corpus 

 callosum is developed to a greater extent in man than in other animals 

 (Fig. 472). This is explained by the fact that in no other animal does 

 mentality reach so high a state of development as in man, and, because 

 the cerebral hemispheres are the seat of mentality, it follows that much 

 greater connection between the cortex of the two hemispheres is needed 

 in man than in other animals. There is a thin translucent membrane 

 between the body of the corpus callosum and the fornix known as the 

 septum pellucidum, which leaves a slight cavity between the two septa 

 of each side. Formerly this cavity was called the fifth ventricle. It has 

 no connection whatever with any of the true ventricles. 



Two tracts of nervous matter run back on the medial side of either 

 hemisphere, from the olfactory lobe to the hinder end of the cerebrum. 

 One of these is the hippocampus, which passes dorsad, and the other 

 is the olfactory tract, which goes ventral to the foramen of Monro. These 

 two and the associated olfactory substances make up practically all of 

 the so-called archipallium in the lower vertebrates, for in these the whole 

 cerebrum really is accessory to the sense of smell. In mammals, and 

 possibly as low as the reptiles, a part has been added to receive impres- 

 sions from other somatic senses. This is the neopallium which has 

 grown out lateral to the hippocampus and is especially large in the higher 

 mammals. In man it forms by far the* greater part of the cerebrum. Its 

 great development forces the olfactory parts to the medial and lower 



