192 



Arachnoid trabecula 



Basic Structure of Vertebrates 



Arachnoid villus 



Dura mater 



Subarachnoid space 



Falx. 



Cortex cerebri 



Fig. 162. Diagram of arachnoid and subdural spaces. (From original by L. H. Weed, 

 Am. J. Anatomy, 31 :202.) 



is organized into a membrane. Outermost of the three is the dura 

 mater, thickest and toughest of all, and more or less closely attached 

 to the periosteum, which is sometimes regarded as a part of the dura 

 mater. 



The dura mater of mammals folds in between the cerebral hemi- 

 spheres to form the falx cerebri. A similar fold between the hemi- 

 spheres and the cerebellum is the tentorium cerebelli. 



The meninges are derived from loose mesoderm (mesenchyme) 

 which surrounds the embryonic neural tube. 



Sense-Organs 



Sense-organs consist of cells or groups of cells adapted to respond 

 to stimuli and transmit impulses to the nerves associated with them. 

 The essential property of a sense-cell is irritability. The stimuli may 

 be caused by physical or chemical agencies either in the environment 

 or within the animal. The conductor may be a nerve-fiber derived from 

 the sense-cell itself, or an afferent fiber whose cell-body is in a ganglion 

 of a cranial or a spinal nerve. 



The more highly elaborated sense-organs possess various structures 



