CHAP. VII.] THE NERVOUS SYSTEM. 87 



and longitudinal white fibres, intermixed with gray matter. 

 The transverse fibres come mainly from the cerebellum, and 

 serve to join its two halves. The longitudinal fibres come from 

 the medulla oblongata. This bridge is a bond of union between 

 the cerebrum, cerebellum, and medulla oblongata. 



The cerebrum is by far the largest part of the brain. It is 

 egg-shaped or ovoidal, and fills the whole of the upper portion 

 of the skull. It is almost completely divided by the median 

 fissure into two hemispheres, the two halves, however, being 

 connected in the centre by a broad transverse band of white 

 fibres, called the corpus callosum. Each half is subdivided into 

 lobes. 



The longitudinal fibres of the medulla oblongata, passing 

 through the pons Varolii, become visible in front of the bridge 

 as two broad, diverging bundles. These two bundles form what 

 are called the crura cerebri, or pillars of the brain, and are situ- 

 ated on the under surface of each hemisphere. Between the 

 crura cerebri is a narrow passage (aqueduct of Silvius) lead- 

 ing from the fourth ventricle into a smaller cavity called the 

 third ventricle. In each side wall of the third ventricle is an 

 opening (foramen of Monro) which leads into two large cavi- 

 ties, the lateral ventricles, and which occupy the centre of each 

 half of the cerebrum. (It will be seen from the above descrip- 

 tion that the cavities in the centre of the brain are continuous 

 with the central canal in the spinal cord, and also that fibres 

 from the cord pass into the centre of the cerebrum.) Forming 

 the floors of the ventricles, lodged in the crura cerebri, and 

 scattered in their neighbourhood, are irregularly shaped masses 

 of gray matter, intricately connected with one another and with 

 the gray matter in the medulla oblongata. The surface of the 

 cerebral hemispheres is folded, the folds or convolutions being 

 deeper and more numerous in some brains than others ; the 

 whole of the convoluted surface is composed of gray matter, 

 i.e. of cell-bodies and naked processes. 



The whole brain appears to consist of a number of isolated 

 masses of gray matter some large, some small connected 

 together by a multitude of medullated fibres (white matter) 

 arranged in perplexing intimacy. But a general arrangement 

 may be recognized. The numerous masses of gray matter in 

 the interior of the brain may be looked upon as forming a more 



