BRAIX. 151 



large and become somewhat pyramidal, the enlargement being due in part to the 

 decussation of the crossed tracts. The tracts pass forward from the decussation 

 and in the mid-brain region they diverge to pass the hypophysial structures farther 

 in front, the diverging portions being called the crura cerebri. The fibres of the 

 crura enter the corpora striata and in the mammals, the cerebral cortex. 



The direct pyramidal tracts have no decussation in the medullary region, but 

 pass to the hemisphere of the same side; the fibres, however, do cross in the spinal 

 cord. Recently attention has been called to Reissner's fibres which occur in all 

 vertebrates, but are relatively largest in fishes. They arise from the roof of the 

 mid- brain, descend to the aqueduct and pass through the fourth ventricle and into 

 the central canal to terminate at various points in the region of the spinal nerves. 

 It has been suggested that they afford a short cut for visual reflexes. Another 

 supposition is that they regulate the flexion of the body. 



Of the numerous longitudinal tracts in the anterior part of the brain 

 the fornix must be mentioned. It appears first in the amphibia and 

 is well developed in the mammals. Its fibres are connected in front 

 with the hippocampus, pass downward through the lamina terminalis 

 to the floor of the third ventricle, where they produce a marked swelling 

 (corpus albicans) on either side of the ventral surface of the dien- 

 cephalon. They ascend from this point to the optic thalami. The 

 passage of the tracts of the fornix through the lamina terminalis and 

 the forward growth of the corpus callosum stretch the lamina into a 

 thin triangular area, the septum pellucidum, and at the same time the 

 callosum causes the lamina to split, the enclosed cavity being called 

 the * fifth ventricle' though it has no relation, physical or mor- 

 phological, with the true ventricles of the brain. 



ENVELOPES (MENINGES) OF THE CENTRAL NERVOUS 



SYSTEM. 



Both brain and spinal cord are surrounded by envelopes (meninges) 

 of connective tissue which support and protect them, and also, by 

 carrying blood-vessels, provide for their nourishment. These meninges 

 become more complicated with ascent in the vertebrate series. The 

 canal of the vertebral column and the cavity of the skull are lined with 

 a layer of connective tissue, the endorhachis, which is really the perios- 

 teum or perichondrium of the skeletal parts and hence not a true meninx. 

 In the fishes (fig. 149) there is a single envelope, the meninx primi- 

 tiva, which bears the blood-vessels and lies close upon the spinal cord. 

 Between it and the endorhachis is a perimeningeal space, somewhat 

 broken by strands of tissue passing from meninx to endorhachis, and 



