152 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 



filled, like all meningeal spaces, with an albumen-containing cerebro- 

 spinal fluid. 



From the urodeles upward there is an increasing division of the 

 meninx primitiva into two layers, a pia mater bearing the blood- 

 vessels and lying close to the cord, and a dura spinalis, separated from 

 the pia by a subdural space, the perimeningeal space now being known 

 as the peridural. In the mammals the pia becomes invaded by cavities 

 separating a delicate arachnoid membrane from its outer surface, so 

 that there is another space, the subarachnoid, in these forms. 



There may be slight differences in the region of the brain in the 

 higher groups where the dura presses against and finally unites with the 

 endorhachis, forming the dura mater of human anatomy, thus obliterat- 

 ing the subdural space. In the mammals and to 

 a less extent in birds the dura mater forms two 

 strong folds. One of these is longitudinal and 

 presses in between the two cerebral hemispheres as 

 a firm membrane, the falx cerebri. The other 

 fold, the tentorium, is transverse, and is inserted 

 between cerebrum and cerebellum. It is occasion- 

 ally ossified and united to the skull. 



THE BRAIN IN THE SEPARATE CLASSES. 



CYCLOSTOMES. The brain is very different in the 

 two classes of cyclostomes. All parts lie in the same hori- 

 zontal plane, the flexures having disappeared, and the 

 /Y-.HL wn l e presents a primitive, almost embryonic appearance. 

 In the lampreys the somewhat slender brain is elongate 

 and its roof is largely epithelial, this extending to the mid- 

 brain, of which only the hinder part is nervous in the middle 

 line. The small cerebral hemispheres are largely com- 



p IG j. ' Brain of P ose( ^ ^ ^ e corpora striata and the dorsal part of the 



Bdellostoma (Princeton, pallium is purely epithelial, the ventricles being well de- 

 2204). o, skeleton of veloped and extending into the olfactory lobes. The 

 olfactory organ, the .11 11 11, 1-11 11 



brain behind 8iis;V-X, P tlc lo ^ es an( * tne medulla are relatively broad, but 

 nerves. the cerebellum is reduced to an inconspicuous fold in front 



of the fossa rhomboidea. 



Authors do not agree regarding the interpretation of some parts of the myxinoid 

 brain. The whole is much broader and shorter than in the other class and is 

 marked dorsally by a groove running the whole length. According to Retzius, the 

 'twixt-brain of Myxine is invisible from above and the cerebellum is large, com- 

 pletely covering the fossa rhomboidea. The cavities are greatly reduced, the 



