THE BRAIN. 135 



that a parietal (Fig. 108, $Z?),a Varolian (#/?), and a cervical 

 (NE) bend may be distinguished. 



While in Fishes and Amphibia the cerebral flexure later be- 

 comes practically obliterated, it persists more or less markedly in 

 the higher types, more particularly in Mammals. In the latter, 

 moreover, the original relation of the parts becomes still further 

 complicated by the large development of the cerebral hemispheres, 

 which grow backwards, and thus gradually come to overlie all the 

 other parts of the brain. This condition of things attains its 

 greatest perfection in Man. Thus from the primitive relations of 

 the various sections of the brain one behind another, they come 



Vtt Ztt 



FIG. 108. CEREBKAL FLEXURE OF A MAMMAL. 



Vff, prosencephalon ; Zff, thalamencephalon, with the pituitary body (H] at its 

 base ; MH, mesencephalon, which at SB forms the most projecting portion of 

 the brain, representing the so-called "parietal bend"; HH, metencephalon ; 

 NIT, myeleneephalon. forming the " cervical bend " (NB) : the " Varolian bend " 

 (BB) arises on the ventral circumference, at the junction between HH and 

 NH ; .ft, spinal cord. 



to lie eventually more upon one another, the thalamencephalon, 

 mid-brain, cerebellum, and medulla oblongata, becoming covered 

 over by the hemispheres. 



MEMBRANES OF THE BRAIN AND SPINAL CORD. 



The enveloping membranes of the brain and spinal cord arise 

 by a differentiation of a connective-tissue layer lying between the 

 central organs of the nervous system and the surrounding skeletal 

 parts. In Fishes, only two membranes are distinguishable, one, 

 the dura mater, lining the inner surface of the cerebro-spinal 

 canal, and the other, pia mater, investing the brain and spinal 

 cord. The latter represents also the arachnoid of higher Verte- 

 brates, which is not here differentiated as a separate membrane. 

 The dura mater conveys vessels to the walls of the cerebro-spinal 

 canal, that is, to the perichondrium or periosteum, while the pia 

 mater, which is much richer in blood-vessels, has to do with the 

 nutrition of the cerebro-spinal axis. The dura mater consists of two 

 lamellae, which, however, only remain distinct along the whole 



