TISSUES OF THE BODY. 341 



muscular substance, excitable though free of nerves, has done much towards 

 the adjustment of that very old controversy in regard to whether there 

 be such a thing as muscular irritability. The termination of sensory 

 nerves in special anatomical structures, such as the Pacinian bodies, or 

 Krause's tactile corpuscles, is also of great interest. 



To return to the ganglion cells : there seems to be among them just as 

 little coincidence between their anatomical variety and physiological 

 difference as among the nerve tubes. The physiological significance, 

 further, of the apolar nervous cells, is still unknown to us ; even the fact 

 of their existence has in it something strange to the physiologist. The 

 unipolar cell, also, which is looked upon as the starting-point of the fibre 

 proceeding from it, should be connected with the cells adjacent to it by 

 commissures. The physiological purpose for which bipolar cells exist is 

 likewise veiled in obscurity. The most comprehensible are the multi- 

 polar elements with their efferent nervous fibres. 



But, although we are at present unable to understand many things in 

 the texture of the ganglion, nevertheless, important points in relation to 

 the motions of organs have been gained by an acquaintance with the 

 smaller ganglionic plexuses discovered in such surprising numbers. We 

 refer to the submucous ganglionic networks, and plexus myentericus of 

 the digestive apparatus. 



Living nervous substance, further, has, like muscle, electromotor pro- 

 perties. 



As to the amount of interchange of matter which goes on in the 

 nervous elements, we are still in the dark. That it is probably consider- 

 able, is indicated by the fact that a fatigued nerve regains, after a certain 

 period of rest, its original power of functionating, and also that ligature 

 of the arteries of a part brings about a rapid paralysis of the motor and 

 sensible nerves supplying the same. The scanty notes of the preceding 

 section likewise contain all that is at present known of the nature of this 

 interchange of matter. 



As to the question, further, how far an anatomical change goes hand in 

 hand with the chemical, or, in other words, how far the nerve tubes and 

 cells may be regarded as persistent structures, or, on the other hand, only 

 destined for a short existence as transitory formations, we are unable to 

 give any answer. The corpuscles and fibres present themselves in far too 

 great variety of form in the adult body for us to be able to separate 

 young, mature, and older elements from one another. 



192. 



The mode of development of nervous tissue in the embryo is one of 

 the most obscure chapters of modern histology. 



That the brain and spinal cord, together with the internal portions of 

 the higher organs of sense, formed from the first of these, are productions 

 of the so-called corneous layer of Remak, is an ascertained fact. They 

 take their rise, in other words, from the cells of the upper cellular layer 

 nearest the embryonic axis. 



On the other hand, the point of origin of the ganglia and peripheral 

 nerves is still unknown to us. We are still unable to determine whether, 

 as is very probable, these parts are productions of the corneous embryonic 

 leaf, or whether, according to one view which is held, they have not 

 originated independently in the middle germinal plate, and only become 

 subsequently connected with the nervous centres. The connection of 



