266 



SOLID PARTS OF ANIMALS. 



brane. The dura mater, which is most external of the three, is 

 thick, firm, and resisting, and consists, in fact, of the two coats ; 

 the outermost one being fibrous, and the innermost serous. It 

 lines the cranium or scull, to which it is attached, while, at the 

 same time, it invests the brain, and sends in processes which are 

 interposed between its different parts. The pia mater, which is 

 in contact with the brain, is a thin lamella of cellular tissue, per- 

 meated by numerous minute capillary arteries. It invests the 

 medulla spinalis as well as the brain, and dips into the sulci be- 

 tween the convolutions of the latter. The arachnoid membrane 

 is smooth and transparent. One part of it invests the spinal 

 cord and the brain, passing over its surface, without dipping into 

 the convolutions. The other lines the dura mater and its several 

 processes with which it is connected. 



The brain occupies the principal part of the cranial cavity. 

 Its superior surface is convex and arched ; and is divided into 

 two equal and similar hemispheres by the duplication 'of the 

 dura mater called the/r. The surface of the brain is render- 

 ed unequal by several depressions and elevations marked upon 

 it The elevations are called convolutions, and are situated be- 

 tween the depressions. The brain itself consists of two substances ; 

 the outermost portion has a gray colour, and is called the corti- 

 cal part, while the innermost portion, which is white, is called the 

 medullary part. The cortical part forms a layer of variable 

 thickness on the surface of the cerebrum and cerebellum. It is 

 found also within the brain ; sometimes it is covered by the me- 

 dullary portion ; sometimes it seems intimately mixed with it ; 

 or the two substances are disposed in alternate layers. 



The first person who attempted to ascertain the structure of 

 the brain by microscopic observations was Leuwenhoek. In the 

 year 1674, he announced that the medullary portion of the brain 

 of a cow was composed of very subtile globules.* Delia Torre 

 stated that the brain consisted of a pulpy matter swimming in a vis- 

 cid and transparent fluid. f According to the microscopical obser- 

 vations of Ehrenberg,t the cortical substance of the brain con- 

 sists of a fine net-work of vessels, in many places containing par- 

 ticles of blood. This net- work is connected with the vessels of 

 the pia mater. Besides this fine net-work, the cortical portion 



* Phil. Trans, xi. 106. f Poggendorf s Annalen, xxviii. 449. 



Ibid, xxviii. 451. 



