VESSELS. gf 



physiological facts, which are widely separated from one 

 another, and how the demonstration of definite morpho- 

 logical elements may at once most essentially contribute 

 to the elucidation of functions, which, without any such 

 data, would be utterly incomprehensible. 



I will omit to speak here of the more intimate struc- 

 ture of the nervous system, because I shall have occasion 

 hereafter to consider it in a more connected form, else 

 this would be the subject which would most suitably 

 come next, seeing that there exist many points of resem- 

 blance in the structure of muscular and nerve-fibres. 

 But in the nervous system we find, in addition, nerve- 

 cells (Ganglienzellen), which connect the individual fibres 

 with one another, and must be regarded as the most 

 important storehouses for all nervous energy. 



Concerning the structure of the vascular system also I 

 will not here treat in detail, but will only say as much 

 as is necessary to give a cursory view of the matter. 



A capillary vessel is a simple tube (Fig. 3, c), in which 

 we have, with the aid of our present appliances, hitherto 

 only been able to discover a simple membrane, beset at 

 intervals with flattened nuclei, which, when seen in the 

 middle of the surface of the vessel, present the same ap- 

 pearance as in the elements of- muscle, only that they usu- 

 ally lie more at the sides, and therefore frequently have 

 an awl-shaped appearance, from their sharp border alone 

 being perceived. It is this, the most simple class of 

 vessels, which we now-a-days solely and exclusively call 

 capillaries, and with regard to them we cannot say that 

 they become wider or narrower by means of any action 

 of their own, but at most that their elasticity renders a 

 certain degree of contraction possible. Nowhere are 

 there to be witnessed in them genuine processes of con- 

 traction or relaxation succeeding it. The discussions 

 which formerly took place with regard to the contracti- 



