91 



present here from certain of the cranial nerves. All of these 

 fibre-systems from iiverse sources are to become relatei to 

 the remarkable mechanism of Reissner's fibre. It is certainly 

 evident that there are here every means for intercommunication 

 between different parts of the nervous system, a switch-boari, 

 so to speak, of extraordinary possibilities; but to this sub- 

 ject »e shall return further on. 



b. The Suoerfisial Vewones. — The outermost layer of 

 the tecturn mesencephali is characterized by the predominance 

 of nerve-fibres and by the feeble development of its neurones. 

 Receiving as it does the fibres of the optic nerve for their 

 first expansion, we should hardly expect a hi^h degree of ner- 

 vous activity here. There are a few scattering nerve-cells 

 present, however, (Pig. 81, s. a. ) . These are quite minute in 

 size, lenticular in form, and are disposed with their long 

 axes tangential to the surface of the brain, as though squeezed 

 into crevices betv»een the mass of nerve-fibres. The internal 

 organization of the cell presents no features which would mark 

 it as having any degree of importance physiologically (Fig. 5?, 

 s. n. ). 



c. The Hiidle Veurones. — The neurones which lie in the 

 middle layer of the tectum are characterized by their larger 

 size and vastly greater numbers as compared with the outer 

 region. In fact, the number is so great that in a section. 



