262 NERVOUS SYSTEM. [SECT. 1 26. 



and their breadth is very various (Stannius in Mull. Arch. 1847, p. 452) ; 

 lastly, the medulla is wholly absorbed, and only pale tubes remain behind. 

 In the rabbit and pigeon, according to Brown-Seqtiard, cut-wounds even of 

 the spinal cord coalesce, and it can scarcely be doubted that the same result 

 may take place in man. Hypertrophy of the nerve-substance itself is not 

 known to occur, although that of the neurilemma has been observed. 

 Yirchow observed a new formation of fine nerves in pleuritic and peritoneal 

 adhesions, and according to him, the grey substance on the walls of the 

 cerebral cavities also appears capable of being regenerated. Yirchow once 

 found an accidental formation of grey and white nervous substance in an 

 ovarian cyst. 



§ 126. Respecting the functions of the nervous system, the 

 following remarks, bearing more immediately on the anatomical 

 facts, may here suffice. With reference to the two elementary 

 constituents of the nervous svstem, anatomical examination shows 

 that all the parts of the nervous system, which minister to its 

 higher functions, contain grey substance in greater or less quan- 

 tity — as the sympathetic, the ganglia of the spinal and cerebral 

 nerves, the cord and the brain itself — while the nerves, acting 

 only as conducting media, contain only nerve-tubes. This sig- 

 nification of the grey substance being accepted, it may be asked 

 further, does it present differences in its structure as well as in its 

 functions. With regard to this point, it is worthy of note, that 

 the largest ganglionic globules exist in places whence motor effects 

 proceed — as in the anterior horns of the cord between the 

 fibres of the anterior roots, in the medulla oblongata at the 

 origins of the motor cerebral nerves, in the cortical substance 

 of the cerebellum, in the pons Varolii and in the cerebral pedun- 

 cles — whilst, on the other hand, the smallest cells are found in 

 the sensitive regions, as in the posterior horns of the cord, 

 the corpora restiformia and corpora quadrigemina. There does 

 not, however, appear to be a constant relation between the size of 

 the cells and the sensitive and motor functions, for both kinds of 

 fibres arise in the ganglia of the cerebro-spinal nerves and of the 

 sympathetic and in the optic thalami ; in the former from larger, 

 the latter from smaller cells. Accordingly, just as in the case of 

 the nerve-tubes, there appear to be large and small motor cells, 

 and also sensitive cells of various dimensions ; a conclusion which 

 is confirmed by comparative anatomy, seeing that the large bipolar 

 cells in fish are obviously sensitive. An essential distinction be- 

 tween sensitive and motor cells, whether they be of the same or 

 different sizes, cannot be demonstrated, and any differences ob- 

 served between these two kinds of cells are not greater than those 



