354 THE NERVE SYSTEM 



of nerve axones, termed association fibers, or this 

 phenomenon is spoken of as association conduction. 



Semor Conduction. This term is used in speaking 

 of the pathways of afferent or sensor nerve fibers in 

 the tracts of the spinal cord, which convey sensa- 

 tions of pain, external temperature, thirst, etc., from 

 the skin, mucous membranes, etc., to the centres 

 in the brain directly or indirectly, which are received 

 by nerve cells in the cortex, giving rise to conscious 

 sensations. These afferent pathways are not thoroughly 

 understood, but the main one is called the crossed 

 pyramidal tract. 



Motor Conduction. This is the term used in speak- 

 ing of the pathways of efferent or motor nerve fibers 

 in the tracts of the spinal cord which convey motor 

 impulses from the cells in the cortex of the brain to 

 centres in the spinal cord, that transmit nerve impulses 

 to the muscles, glands, etc., and promote their activity. 

 The main motor pathway is called the direct pyramidal 

 tract. 



THE ANATOMY AND PHYSIOLOGY OF THE 

 BRAIN 



The Brain (Encephalon). The encephalon or brain 

 is that part of the cerebrospinal system which, with 

 its membranes, is contained in the cranium. It is 

 composed of the cerebrum, cerebellum, pons Varolii, 

 and medulla oblongata. 



The Membranes of the Brain. These are the dura, 

 the pia, and arachnoid. 



The dura mater is similar in structure to the dura of 

 the cord, but differs from it in being closely attached 

 to the cranial bones, forming, in fact, their inner peri- 

 osteum. It is continuous with that of the cord at the 

 foramen magnum, and with the external periosteum of 



