566 Dynamic Theory. 



up into a sort of band, the corpus firabriatum or taenia hippocampi, the 

 so-called posterior pillar. Other fibres pass backwards into the posterior 

 lobes ( 3 ), the posterior cornu, and hippocampus minor, while still 

 others, crossing over the hippocampus major (16) and its "pes" ( 17 ), 

 connect with the middle lobe. The fornix is white throughout, except 

 a grayish tint at its summit. 



The brain is enclosed by three membranes called the pi a mater, the 

 arachnoid and the dura mater. The pia mater lies next the brain, 

 is very delicate, full of small blood vessels, and follows all the convo- 

 lutions and depressions. The arachnoid lies outside the pia mater, and 

 consists of two layers. They do not follow the pia mater into the sin- 

 uosities of the convolutions, but the interior layer penetrates to the un- 

 derside of the hemispheres, and lines the cavities therein. The dura 

 mater lies outside the arachnoid next the skull. It is pearly white, 

 semi-transparent, thick, and very strong. These membranes envelop 

 not only the brain but also the spinal cord. Outside of the brain, be- 

 neath the arachnoid membrane, is the ( l cerebro-spinal fluid," averaging 

 about two ounces. It serves as a cushion to keep the pressure on the 

 brain cells equal. If the brain rapidly increases in size by growth of 

 new tissue, or turgescence, the amount of fluid decreases, while, in case 

 of atrophy, it has increased to as much as twelve ounces. 



The pia mater furnishes to the anterior parts of the brain, appendages 

 of vascular tissue, which furnish a vehicle for the circulation of the 

 blood. A leaf of this tissue from the pia mater, called the choroidea 

 tela, lines the lower surface of the body of the fornix, covers the cor- 

 pora quadrigemina, the posterior commissure and optic thalamus, sepa- 

 rating the latter body from the cornu ammonis. Thence a prolongation 

 of it extends into each descending cornu on the anterior side of the hip- 

 pocampus major. This appendage is called the choroid plexus. It is 

 attached by one edge to the tela choroidea, while the other floats loose. 

 Two veins traverse the tela choroidea, into which most of those about 

 the lateral ventricles and from the upper part of the cerebellum, the 

 pineal gland and the corpora quadrigemina, empty. They are called the 

 vence galeni. These pass backward and often, first uniting into one, pass 

 on under the splenium of the corpus callosum, into the straight sinus, 

 &c. , the blood finally reaching the jugular veins. The silent activities 

 of the brain are large consumers of blood. 



The right and left choroid plexuses are joined together at their an- 

 terior ends by a cord which passes under the arch of the fornix across 

 the foramen of Munro. 



The Ventricles. Some further description of the ventricles, showing 

 their connection and relations to each other, is desirable. First, there 

 is a cylindrical canal of small diameter, which traverses the center of 



