ELEMENTS OF NERVOUS TISSUE. 69 



The ventricles of the brain and the aqueduct of Epithelium of 



the ventricles. 



Sylvius, which are properly to be regarded as the 

 continuation, in the cranium, of the central canal of the 

 spinal cord, are, like it, lined by a layer of ciliated 

 epithelium. This epithelium, in some cases, is truly 

 cjliated only in the fourth ventricle ; at least this is 

 to be inferred from the researches of Leydig upon 

 the brain of a criminal. {Gazette hebdomadaire, 1854,. 

 "p. 687). Beneath the epithelial layer there are some 

 fibrillse of connecting tissue forming an exceedingly 

 delicate lamina, in which amyloid corpuscles are 

 found. (Virchow.) 



The membranes by which the cerebro-spinal ner- ^S br a?dB mai 

 vous centres are enveloped, are composed of con- cord< 

 nective and elastic fibres, woven together in layers of 

 different degrees of density. They have but few 

 blood-vessels of their own, and still fewer nerves ; as 

 to lymphatics, their existence has not even been 

 demonstrated. The external surface of the visceral 

 layer of the arachnoid is invested with pavement 

 epithelium, which passes from it upon the free sur- 

 face of the dura mater, where it alone forms the 

 parietal layer of this serous membrane. 



The corpuscles of Pacchioni, found in the dura corpuscles of 



Pacchioni. 



mater along the course of its great longitudinal sinus, 

 consist of very dense connecting tissue, enclosing 

 sometimes, in its meshes, amyloid corpuscles and cal- 

 careous concretions.* 



Kolk, Professor in the University of Utrecht, published by the Eoyal 

 Academy of Sciences at ^isterdam, and translated and republished by 

 the New Sydenham Society, London, 1859. (Ed.) 



* The pineal gland contains pale rounded cells_without any processes, 



