STRUCTURE OF THE VERTEBRATES 293 



INIyelencephalon. The posterior part of the hindbrain is the 

 medulla oblongata which is continuous with, and most resem- 

 bles, the spinal cord. The ventricle of the medulla is continuous 

 with that of the cerebellum, and is the large fourth ventricle 

 of mammalian anatomy. The roof is thin and covered with a 

 plexus of nutrient vessels which usually dip into the ventricular 

 space. 



The medulla is roughly triangular in shape, the ventral region 

 being filled with the large tracts of nerve fibers which pass from 

 the cord to the upper centers of the brain. In external view the 

 most prominent are the pyramidal tracts, which are large 

 bundles on the ventral surface. Other tracts and nuclei occupy 

 the lateral expansions, and these nuclei are relay centers for 

 impulses to the cerebellum and cerebrum. The posterior limit 

 of the medulla is an artificial division from the cord, determined 

 by the foramen magnum. 



Commissures. A commissure is any large group of fibers 

 crossing from one side of the brain to the other. Every impulse 

 entering the body crosses to the opposite side before reaching 

 the upper centers of the brain. Thus, the left half of the brain 

 influences the right side, and the right influences the left. This 

 is due to the decussation, or crossing, of the fibers. The fibers 

 may cross at any level of the brain or cord, but the decussations 

 in the cord are isolated and few in number. More generally the 

 fibers from receptors pass up the cord on the original side of 

 entrance, and cross in the brain stem. 



The pons of the cerebellum is a large commissure, and others 

 have been mentioned. In the cerebral region there are several 

 commissures connecting one side with the other. The most an- 

 cient in a phylogenetic sense is the anterior commissure which 

 lies in front of the third ventricle in the lamina terminalis. The 

 amphibia develop a commissure dorsal to this one, the pallial 

 commissure, connecting the hippocampal regions. Another de- 

 velops in this region in the higher mammals, the corpus callosum. 

 This large body of fibers lies dorsal to the ventricle and in 

 longitudinal sections gives the appearance of a strong band of 

 connective tissue. The corpus callosum has not been described 

 in the monotremes; it is very weak in the marsupials; and in- 

 creases in size in mammals with a well developed cerebral cortex. 



