THE BRAIN IN THE SEPARATE CLASSES 1 63 



perimeningeal space, somewhat broken by strands of tissue passing 

 from meninx to endorhachis, and filled, like all meningeal spaces, with 

 a cerebrospinal fluid containing albumen. 



From the urodeles upward there is an increasing divison 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 cavi- 

 ties separating a deUcate arachnoid membrane from its outer surface, 

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



There may be shght 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 

 obliterating the subdural space. In the mammals and to a less ex- 

 tent in birds the dura mater forms two strong folds. One of these is 

 longitudinal and presses into the longitudinal fissure between the two 

 cerebral hemispheres as a firm membrane, the falx cerebri. The 

 other fold, the tentoriimi, is transverse, and is inserted between cere- 

 brum and cerebellum. It is occasionally ossified and united to the 

 skull. 



The Brain in the Separate Classes 



CYCLOSTOMES. — The brain differs greatly in the two subdivisions of 

 cyaostomes. In both the flexures (fig. 170) are never as marked as in the 

 gnathostomes and are entirely lost in the adult, where the brain has a primitive, 

 almost embryonic appearance. 



In the lampreys (fig. 170) the brain is long and slender, and its roof is epi- 

 thelial, even in the mid-brain region, there being large everted chorioid plexuses 

 in the roof of mid-brain and medulla (removed in the figures). The small 

 telencephalon is indistinctly divided into so-called olfactory lobes and cerebral 

 hemispheres, but in reality olfactory bulbs in front and olfactory lobes behind, 

 the whole being subsidiary to the sense of smell. There are two epiphysial out- 

 growths from the roof of the 'twixt-brain (removed in the figure). The posterior 

 (upper) of these is clearly the pinealis, but the homologies of the other are un- 

 certain and it has been called the parapinealis. The weakly developed infun- 

 dibulum has associated with it a median sac, probably representing the vascular 

 sacs of the ichthyopsida. This is closely connected with the hypophysis, which 

 develops, not from the oral (ectodermal) epithelium as in other vertebrates, but 

 by an inpushing (fig. 212) between nasal and oral invaginations (see also p. 157). 

 Mid- and hind-brains are relatively broad, and the cerebellum is reduced to a 

 narrow cross band, much like that of amphibia. The ventricular cavities are 

 large and well developed. 



The myxinoid brain (fig, 171) is shorter and broader and is marked by a 

 groove running the whole length of the dorsal surface. The medulla oblongata 



