NERVOUS SYSTEM 839 



Cyclostomes are the only vertebrates whose brains remain in this 

 simple three-chambered state. In all other forms there are many modifi- 

 cations of the primitive brain, though no matter how many modifica- 

 tions there may be, they all form as ingrowths or outgrowths of this 

 primary type. 



The prosencephalon divides into an end-brain or telencephalon con- 

 sisting of the cerebral hemispheres, and the twixt-brain or diencephalon 

 consisting of the thalamus and the hypothalamus. Each of these in turn 

 again divides, forming the parts enumerated in the accompanying table. 

 (Pages 840, 841). 



The mesencephalon divides into four lobes (in mammals called 

 corpora quadrigemina and in lower forms of vertebrates, where these 

 bodies have not again divided transversely, corpora bigemina or optic 

 lobes), and cerebral peduncles. 



The rhombencephalon is made up of the isthmus rhombencephali 

 (consisting of the superior cerebellar peduncles, the anterior medullary 

 velum, the trigonum lemnisci and the crura cerebri, the isthmus itself 

 connecting mesencephalon and rhombencephalon), the metencephalon 

 (consisting of cerebellum and pons), and the myelencephalon or medulla 

 oblongata. 



THE CEREBRUM 



The prosencephalon and the mesencephalon together are often called 

 the cerebrum. 



* The greater part of the telencephalon is made up of the two hemi- 

 spheres which are divided by a longitudinal fissure. This fissure is not 

 well marked in fishes, but is very distinct in other groups of animals. 

 The lateral ventricles are contained one in each hemisphere, while a part 

 of the third ventricle (commonly called the foramen of Monro), (Fig. 

 303), lies between the two. The corpus striatum is a ganglion mass 

 lying upon the floor of the lateral ventricle, while the cortex of the hemi- 

 spheres is called the pallium.* The substance of the hemispheres varies- 

 to a considerable extent in the different types of animals. In fact, in. 

 the fishes it is practically all pallium, for there is merely a thin non-r 

 nervous covering to the ventricles. In reptiles and birds the gray matter 

 (nerve cells) is to be found on the ventricular surface, while the outer 

 surface is composed of white matter (fibers). In the reptiles there is the 

 beginning of a second layer of cells a little distance from the ventricular 

 surface. In birds this is still further increased, while in mammals there 

 is a complete layer called the cerebral cortex over almost the entire sur- 

 face of the hemisphere. 



*The cortex merely means an outer portion, and, in the brain, is usually composed of grey matter, 

 while the pallium is merely the outermost covering of the hemispheres, whether composed of grey 

 matter or not. 



