BRAIN. 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 
