MEDULLA OBLONGATA. 157 



ginating by two roots ; the anterior from the anterior column, and 

 the posterior from the lateral fissure, where they connect with the 

 posterior horns. The poste- 

 rior is the larger, and upon it ^^S- 139. 

 is a ganglion, which is in the 

 intervertebral foramen, beyond 

 which the two roots unite. 

 These two roots are separated 

 by the ligamentum denticu- « 

 latmn, a process of pia mater, 



having a serrate appearance, which contains a good deal of yellow 

 fibrous tissue in its structure. 



MEMBRANES OF THE BRAIN. 



The brain has three membranes, of which the most external is the 

 dura mate?-. It is of the same structure as the dura mater of the spinal 

 cord, composed almost entirely = of white fibrous tissue. It consists 

 of two laminee, which are only separate at the points where the 

 sinuses exist. 



It adheres firmly to the bones of the cranium, particularly to those 

 at the base of the skull. The internal lamina forms several pro- 

 cesses. The falx cerebri is a fold of the internal lamina, reaching 

 in the median line from the crista galli to the tentorium, and sepa- 

 rating the two hemispheres of the brain. The tentorium is another 

 fold, stretched horizontally, and also of a crescentic shape, separating 

 the cerebrum from the cerebellum. The falx cerebeUi is a smaller 

 fold, reaching from the internal occipital cross to the foramen mag- 

 num, and separating the lobes of the cerebellum. 



The arachnoid is the serous membrane covering the brain, and 

 lining the inner surface of the dura mater. It is thin and trans- 

 parent. 



The ^9ia tnater is a vascular covering consisting of arteries and 

 veins united by a small quantity of cellular tissue. It passes down 

 in the fissures, between the convolutions, so as to invest them com- 

 pletely. It contains a number of g/a?^r/5. o/' PaccAaiom, which are 

 usually small and white, but which sometimes are very large, espe- 

 cially in old persons. 



m 



THE BRAIN. 



The brain consists of four principal parts : Medulla Oblongata, 

 Pons Varolii, Cerebrum, and Cerebellum. 



MEDULLA OBLONGATA. 



This is the upper part of the spinal cord ; it is conical in its shape, 

 and extends from the atlas to the pons Varolii. It is divided by the 



14 



