THE BRAIN. 127 



What is the Meso-cephalon ? It comprises the parts which connect the 

 cerebrum with the cerebellum and the medulla oblongata, and includes the 

 following structures, viz : 



Pons Varolii, a great transverse commissure seen at the base of the brain 

 in front of the medulla. Its fibres connect the hemispheres of the cere- 

 bellum with each other and the medulla. 



Crura Cerebri, or Peduncles of the Cerebrum, extend from the pons to the 

 corpora striata and optic thalami, and consist of the Peduncular Fibres 

 which pass from the medulla to the ganglia named, and thence upwards 

 to the gray matter of the hemispheres as the Corona Radiata. Each crus 

 contains in its centre a mass of gray matter called the locus niger. 



Valve of Vieussens, is a thin lamina forming the roof of the iter e tertio ad 

 quartam ventriculum, and stretched between the two processus e cerebello 

 ad testes. Its Frenulum is a ridge descending on its upper part from the 

 corpora quadrigemina. 



Corpora Quadrigemina, or Optic Lobes, are 4 spherical eminences placed 

 together above the valve of Vieussens, and behind the 3d ventricle be- 

 neath the posterior border of the corpus callosum. The anterior pair are 

 called the Nates, the posterior, the 7esfes, and they are connected with the 

 optic thalami and optic tracts by 4 bands, their Brachia ; and with the 

 cerebellum by a white cord on each side, the Processus e Cerebello ad Testes. 

 They form two ganglia for the centre of vision. 



Pineal Gland, is a small conical reddish body situated between the nates, on 

 which it rests. Its 4 peduncles connect it with the anterior crura of the 

 fornix, and the optic thalami. It is very vascular, and has a small cavity 

 (said to communicate with the 3d ventricle) which contains a viscid fluid, 

 and some concretions formed of the phosphates of lime, magnesia, and 

 ammonia, etc., called the Acervulus Cerebri. 



Describe the Medulla Oblongata. This ganglion is the upper enlarged 

 part of the spinal cord, extending from the upper border of the atlas to the 

 pons Varolii. Its posterior surface forms the floor of the 4th ventricle, its 

 anterior surface rests on the basilar groove of the occipital bone. It is divided 

 into two lateral halves by the Anterior and Posterior Median Fissures, and con- 

 tains gray matter scattered throughout it, from which the cranial nerves from 

 the 5th to the I2th inclusive arise entirely or in part. In it are supposed to lie 

 the centres for the vaso-motor and the cardiac nerves, also centres of respira- 

 tion, phonation, deglutition, mastication, and expression. On each side it pre- 

 sents the following, 



Anterior Pyramid, formed by the anterior and lateral columns of the spinal 

 cord. 



