378 HEALTH AND DISEASE 



backwards of the lumbar division. It presents four long ganglia, which 

 communicate with the inferior sacral nerves by a few small filaments. 



This plexus distributes its branches to the coccygeal artery, and in 

 a somewhat irregular manner to other neighbouring parts. 



Structure of the Cerebro-spinal Nervous System.— If a fragment 



of the brain or spinal cord be examined with the unassisted eye, it appears 

 to be composed of a soft curd-like material with red points and streaks 

 distributed irregularly through it, differing a little in colour in different 

 parts, being here almost pure white and there pinkish gray, but every- 

 where so soft and apparently destitute of structure, that Haller, one of the 

 most learned and expert physiologists of the eighteenth century, could 

 only describe it as a uniform pulp with but few indications of structure, 

 presenting only blood-vessels and some obscure fibrous markings. The 

 great improvements that have been made in the construction of the 

 microscope, and in the process of hardening, cutting, staining, and mount- 

 ing specimens for microscopic examination, now enable it to be shown 

 that whilst there is an abundant supply of blood circulating through each 

 part, the essential elements of every nervous system are nerve-cells and 

 jierve-fihres, both of which require careful consideration. 



The Nerve-cells. — The nerve-cells are bodies of rounded, oval, or 

 irregular form, varying greatly in size, l)ut always microscopic, and having 

 an average diameter of about 1 -2000th of an inch. Each cell contains in 

 its interior a small but important structure, named the " nucleus", lying- 

 in a mass of finely fibrillated protoplasm, and itself containing a still 



