838 COMPARATIVE ANATOMY 



more cephalic portion of the medulla oblongata where the trigeminal and 

 facial nerves arise. From the sixth the glossopharyngeal nerve arises, 

 while the vagus is directly connected with the remaining two. 



Meninges. 



In examining any brain, one finds, after the bony parts of the skull 

 have been carefully removed, a connective tissue envelope lying close to 

 the bone. This is called the endorhachis, but is really the periosteum or 

 perichondrium of the bony parts and not a true envelope of the brain. 



In the ascending groups of vertebrates we find a more complex 

 arrangement of brain and spinal-cord-envelopes. It must be understood 

 that what is here said of the brain-coverings proper, must also be said 

 of the entire spinal cord. 



In the fishes there is but a single covering envelope called the 

 meninx primativa. The blood vessels are carried within this meninx. 

 There is an open space between this meninx and the endorhachis called 

 the perimeningeal space filled, as are all such spaces, with the cerebro- 

 spinal fluid. Tiny strands of tissue pass between the two connective 

 tissue layers. 



In the urodeles, and from there on upward in the various phyla, the 

 meninx has two layers, namely the pia mater, which bears the blood 

 supply and lies close to the neural cord, and the dura spinalis or dura 

 mater. A space between these two layers is called the subdural space, 

 while the perimeningeal space is then called peridural. 



In mammals the outermost layer of the pia mater again separates 

 from the pia proper, becoming a delicate arachnoid layer, and the space 

 thus formed is called the subarachnoid space. 



In man and some of the higher groups of.-animals the dura spinalis 

 unites with the endorhachis, obliterating the subdural space, and this 

 united sheet of covering is called the dura mater. This dura mater forms 

 two strong folds in the mammals, and to a small extent in birds, and 

 presses longitudinally into the longitudinal fissures separating the two 

 hemispheres of the brain. It is then known as the falx cerebri. The 

 other fold presses transversely between cerebrum and cerebellum form- 

 ing the tentorium. Sometimes these folds even ossify and unite with the 

 skull. 



THE BRAIN. 



The forepart of the spinal cord becomes constricted in two places 

 transversely, forming three divisions, each being hollow in the center 

 (Fig. 288.) 



Starting with the cephalic end (Fig. 471), the first compartment thus 

 formed is known as the fore-brain or prosencephalon. The central por- 

 tion forms the mid-brain or mesencephalon while that portion extending 

 caudally is called the hind-brain or rhombencephalon. 



