PHYSIOLOGY. 15 



spinalis ; and of the same substance continued into the nerves, 

 by which it is distributed to many different parts of the body. 



XXIX. The whole of this system seems to be properly dis- 

 tinguished into these four parts. 



1. The medullary substance contained in the cranium and 

 vertebral cavity, the whole of which seems to consist of distinct 

 fibres, but without the several fibres being separated from each 

 other by any evident enveloping membranes. " This (the 

 fibrous structure of the medullary substance) is a necessary part 

 of anatomy. It may not be evident in many parts of that 

 substance : the arrangement, however, is more evident in the 

 brain of some animals than in the human brain ; but by boil- 

 ing, and thereby hardening the brain, it becomes evident in 

 every one. Also, in certain cases of hydrocephalus, where the 

 small portions are separated by the water, the fibrous structure 

 appears. I say, ( the whole seems to consist of distinct fibres, 

 but without the several fibres being separated from each other 

 by any evident enveloping membranes ;' but there is a suspi- 

 cion that they are kept distinct by some substance which may 

 be considered as an enveloping membrane." 



Note. When we speak of functions, which are or may be in 

 common to every part of this portion of the nervous system, we 

 shall speak of the whole under the title of the BRAIN ; but, 

 when it is necessary to distinguish the particular parts, we shall 

 take care to avoid ambiguity. 



2. Connected with one part or other of No. 1. are the NERVES, 

 in which the same medullary substance is continued, but here 

 more evidently divided into fibres, each of which is separated 

 from the others by an enveloping membrane derived from the 

 pia mater. 



3. Parts of the extremities of certain nerves (2.) in which 

 the medullary substance is divested of the enveloping mem- 

 branes from the pia mater, and so situated as to be exposed to 

 the action of certain external bodies, and perhaps so framed as 

 to be affected by the action of certain bodies only : these we 

 name the SENTIENT EXTREMITIES of the nerves. 



4. Certain extremities of the nerves (2.) so framed as to be 

 capable of a peculiar contractility, and, in consequence of their 



