274 



STRUCTURE OF THE VERTEBRATES 



the chordates. The cavity remains small in the spinal cord, but 

 as the brain grows the neurocoel expands and becomes en- 

 larged into ventricles. Each region carries a ventricular cavity 

 with it, but the ventricles of the brain are connected with each 

 other and with the neurocoel of the cord. The ventricles are 

 fluid filled and help supply the inner surfaces of the brain with 

 oxygen and food. 



The brain and cord are enclosed by connective tissue cover- 

 ings, the meninges of the central nervous system. In the embryo 

 this is a single meninx, closely adherent to the nerve cord, 

 which carries blood vessels to the brain. The space between the 

 meninx and the periosteum of the skull and neural arches 



I 



Fig. 155. Comparative Anatomy of the Meninges. The single dura spinalis 



of the dogfish (A) becomes double in the reptiles. A third membrane, the 



arachnoid, appears in the mammal (C). 



is filled with fluid, and the two membranes are connected by 

 fibers which pass freely between the two. This simple covering 

 is retained by the fishes throughout life. Beginning with the 

 amphibia there is an increasing complexity of the meninges. The 

 meninx separates to form (1) a thin covering of the brain, the 

 pia mater; and (2) a heavier outer layer, the dura spinalis. In 

 the mammals the pia divides, a microscopically thin arachnoid 

 layer being added as a meninx. The dura spinalis becomes the 

 dura mater. This membrane lies against the periosteum of 

 the bone and the two eventually (about the eighth year in the 

 human) become inseparably united. The spaces between the 

 membranes is always filled with fluid which is protective and 

 nutrient. The meninges pass out over the spinal and cranial 

 nerves, enclosing them as a sheath. 



