COMPARATIVE ANATOMY OF THE NERVOUS SYSTEM 357 



Returning to the medial sagittal section, identify in the roof of the midbrain 

 the two hillocks formed by the superior and inferior colliculi or corpora quad- 

 rugemina. Below them is a narrow passage, the aqueduct of the brain, which 

 connects the third ventricle in the diencephalon with the fourth ventricle in the 

 medulla. Below the aqueduct is the thick floor of the midbrain, the tegmentum. 

 At the sides of this are the cerebral peduncles, not exposed in the section. In 

 the section of the cerebellum note the curious branching treelike arrangement 

 of the white matter, forming the arbor vitae or tree of life. This appearance is 

 brought about by the fact that each fold of the cerebellar surface consists of gray 

 matter or nerve cells, with a central plate of white matter or nerve fibers. The 

 cerebellum fits into the fourth ventricle from which, however, it is separated in 

 the normal condition by a membrane, the medullary velum. Part of this velum 

 will probably be found below the anterior part of the cerebellum. Identify 

 in the section the mass formed by the pons. The section of the medulla has 

 nothing of additional interest. 



Draw the sagittal section. 



e) Further structure of the cerebral hemispheres: On the intact half of the brain 

 begin to cut away the roof of the cerebral hemisphere in thin slices. Note that 

 the superficial substance of the roof is gray, the interior white, a reversal of the 

 condition found in lower vertebrates. This gray outer coat of the mammalian 

 brain is called the cortex; it is gray because it consists of nerve cells which have 

 migrated to the surface from their original, more central position. The white 

 matter consists of fibers which carry impulses to and from the cortex. In the 

 cat the cortex is much convoluted. Note that each convolution has a central 

 core of white matter and a peripheral thick coat of gray matter. Continue to 

 shave the brain ventrally until the corpus callosum is exposed. This is a narrow 

 band of fibers conveying impulses between the two hemispheres. Remove the 

 corpus callosum and the cortex lateral to it. This exposes the cavity of the cere- 

 bral hemisphere, called the lateral ventricle. It is filled by two conspicuous 

 elevations. The anterior and smaller one, of a darker color, is the corpus striatum, 

 a mass of gray matter. The posterior, larger one is the hippocampus. Remove 

 the side of the hemisphere so as to expose the hippocampus. It is a curved body 

 with an anterior free margin, the fimbria. Cut through the medial attachment 

 of the hippocampus, raise the anterior border, and roll the hippocampus back. 

 Observe that the part of the hippocampus still attached is continuous with the 

 pyriform lobe. The hippocampus is a part of the original external surface of 

 the brain, which has been invaginated into the interior. The turning back of 

 the hippocampus reveals the thalamus Note the thick stalk extending from the 

 thalamus into the cerebrum, immediately in front of the pulvinar and lateral 

 geniculate body. Scrape the surface of this and note that it consists of a great 

 mass of fibers radiating from the thalamus into the cerebral hemisphere. This is 



