NERVOUS SYSTEM. 57 



and mid brains. The backward extension of the cerebrum is es- 

 pecially marked in mammals. Connected with this overgrowth 

 is the formation of the fifth ventricle, or pseudo-ventricle, a 

 cavity in no way connected with the true ventricles, but lying 

 morphologically outside the brain, between the septa pellucida, 

 the fornix, and the corpus callosum. 



The brain and spinal cord are enclosed in envelopes of 

 mesenchymatous origin, which hold them in position, and serve 

 as the bearers of nutrient vessels, etc. These membranes from 



FIG. 59. Sagittal section through the head of pig embryo of 15.5 mm. length, 

 showing the cranial flexures. AA, axis of brain; C\ cephalic flexure; //, hypophy- 

 sis ; 7/7', heart ; J\f, mouth ; 7', pontal flexure ; 7', tongue. The nervous tissue dotted. 



outside to inside are the dura mater and the pia mater. Of 

 these the dura is a more dense connective tissue, consisting of 

 two lamellae in the lower vertebrates ; its blood-vessels being 

 distributed to the walls of the spinal canal and the skull. The 

 pia is more delicate, and bears the blood-vessels of the brain and 

 cord. Between the two layers is a large lymph space, and in 

 the amphibia and higher vertebrates this is divided by a third 

 membrane, the arachnoid. The pia enters all the fissures 

 and depressions in the brain and cord, carrying nourishment 

 into the nervous mass. 



