202 



The Vertebrate Organ Systems 



spiderwebby tissue which passes over the convolutions of the brain 

 without dipping into them. The outermost layer, the dura mater, is 

 composed of heavy fibrous connective tissue. It adheres closely to 

 the inside of the skull. Between the arachnoid and pia mater is a space 

 filled with cerebrospinal fluid derived from the connections with the 

 ventricles. This fluid serves as a pad to cushion the central nervous 

 system from shocks. An inflammation of any of these meninges is 

 known as meningitis. 



OtrACTORY LOBE 



TELENCEPHALON 



CEREBRAL 

 HEMISPHERE 



DIENCEPHALON 

 PINEAL BODy 



OPTIC LOBE 



MESENCEPHALON 

 AQUEDUCT, 



METENCEPHALON 



CEREBELLUM l^ 



MYELENCEPHALON. 



Fig. 62. — Diagrams of vertebrate brain showing regions and ventricles. 

 A, Dorsal view; B, frontal section. 



The Central Nervous System: the Spmal Cord. — The spinal 

 cord has a uniform appearance throughout its length. At the anterior 

 end where it is connected with the medulla, the cord is widest. Gradu- 

 ally it tapers posteriorly, terminating in a thread-like process, the 

 filum terminale. There are but two slight enlargements along the 

 length of the cord : the most anterior of these is the thoracic enlarge- 

 ment where the large nerves of the arms and shoulders arise, and the 

 lumbar enlargement where the large nerves of the legs arise. 



A cross section (Fig. 63) through the cord shows the arrange- 

 ment of the parts. In the center is a round opening, the lumen of 

 the canal of the cord which is continuous with the fourth ventricle of 

 the myelencephalon. Like the ventricles of the brain, this lumen is 

 filled with the cerebrospinal fluid. Around this opening is an H-shaped 



