OF THE STRUCTURE OF VESSELS. 283 



incurable morbid alterations in important tissues and 

 organs are thus brought about. It is, therefore, of 

 essential importance for us to consider the structure of 

 the vessels and the manner in which they act. This 

 information is the first step towards a conception of 

 the physiological changes which are occurring in man 

 and the higher animals during each moment of life, 

 and without it, it is not possible to form a correct 

 notion of the simplest general pathological change. 



293. Importance of a knowledge of structure of 

 vessels. Without a knowledge of the structure and 

 action of the vessels it is not possible to form an 

 accurate notion even of the process of nutrition as 

 it is carried on in man and the higher animals. 

 Not only is the equable distribution of nutrient fluid 

 dependent upon a healthy state of the organs of the 

 vascular system, but the blood cannot retain its, nor- 

 mal composition for long if the structure of the little 

 vessels through which it passes be modified in any great 

 degree. Hitherto little attention has been paid to the 

 changes in character of the walls of the capillaries 

 in various conditions, but there are few matters of 

 greater importance in connection with the study of 

 disease and the determination of the precise alterations 

 which precede the irrecoverable loss of normal struc- 

 tural characters. 



294. Views regarding the structure of the capil- 

 laries. A capillary has been regarded as a simple 

 elastic tube, the walls of which are readily permeable 

 to fluid in both directions, which act, in fact, as a sort 

 of filter, permitting nutrient matter in solution to flow 

 through from the blood; and the products of the dis- 

 integration and decay of tissues dissolved in fluid, to 

 flow in the opposite direction towards and into the 

 blood. This view was in harmony with the general 

 physico-chemical doctrines taught with such zeal in 

 all departments of physiology a few years ago, when 

 nutrition was regarded as a process akin to crystal- 



