THE NERVOUS SYSTEM. 797 



though less frequently thaii iu those of the brain. As in the brain, when 

 they involve terminal arteries, they determine areas of softening and 

 consequent secondary degenerations. 



LESIONS OF THE VESSEL WALLS IN THE BRAIN AND CORD. 



The mural changes in the vessels are not peculiar to the nervous sys- 

 tem : the most frequent is that known as atheroina or arterio- sclerosis. 

 It is especially common in the system of arterial trunks which make up 

 the circle of Willis. Fatty degeneration is often found in the vessel 

 walls coincident with the increase in the connective-tissue elements. 

 Calcification may occur and may be so extensive that the whole or a 

 greater part of the circle of Willis is converted into a series of hard 

 tubes usually somewhat larger than normal. More commonly the cal- 

 careous areas are irregular in their distribution, giving to the vessels a 

 nodular appearance. Fatty degeneration may also occur, though more 

 rarely, as an independent lesion. It affects particularly the muscular 

 coat. Hyaline degeneration may also occur either as an independent 

 lesion, which is quite frequent in the brains of idiots, or as the initial 

 lesion of a sclerosis. 



DEGENERATION AND INFLAMMATION IN THE BRAIN, SPINAL 

 CORD, AND NERVES. 



Review of Normal Morphology. 



While the scope of this work does not warrant a detailed description of the mor- 

 pltology of the nervous system, 1 our appreciation of the pathological changes to which 

 it is subject is so dependent upon an accurate knowledge of its morpholog}', that, 

 before proceeding further, a brief resume of some of the more fundamental points in its 

 structure aed architecture cannot wisely be omitted. 



The studies of Golgi and of his successors in the use of his technique have led to the 

 so-called neurone conception of nervous-system structure. According to this view, the 

 neurone represents the structural unit of the nervous system. The neurone is the nerve 

 cell with all its prolongations, and the nervous system in toto is but an orderly associa- 

 tion of an immense number of these neurone units. Although these neurones differ 

 from one another as to details of structure they still present an essential similaritj'. 



The neurone is first distinguishable as a small round cell in the epiblastic lining of 

 the embryonic neural canal. Such a cell is entirely devoid of processes. It soon, 

 however, becomes pyriform, and from the tip of the pear grows out a process, which is 

 the axis-cylinder process or axone. Later other processes appear as outgrowths of the 

 cell body. These are the protoplasmic processes or dendiites. Each adult neurone 

 then consists of a cell body, and passing off from this cell body two kinds of processes 

 (Figs. 523 and 524). 



The Cell Body. Our knowledge of the internal structure of the nerve cell has been 

 greatly increased in the last few years by the application of a special technique devised 



'For this the reader is referred to such works as "The Nervous System," by 

 Barker, and the " Anatomic du Systeme Nerveux de 1'Homme," by Van Gehvchten. 



