364 



THE MICROSCOPE. 



terminate and the minute veins commence cannot be exactly defined : 

 the transition is gradual; but the intermediate network is so far peculiar, 

 that the small vessels which compose it maintain nearly the same size 

 throughout ; they do not diminish in diameter in one direction, like 

 arteries and veins; hence the term capillary, from capillus, a hair. 

 (Fig. 175.) The size of the capillaries is proportioned in all animals to 

 that of the blood corpuscles; thus, amongst the reptilia, where the 

 blood corpuscles are the largest, the capillaries are also the largest : but 

 it does not follow that they should be always of the same size in all 



fig. 174. 



1. Blood-vessels of the Eye ; back view of the Iris and ciliary processes. 2. Vessels of 

 the membrana pupillaris from the eye of a Kitten. 3. Fibres or tubes from the 

 lens of the Ox. 



the tissues of one and the same animal ; for if we examine and care- 

 fully measure in the human subject their sizes in different tissues, we 

 shall find that they vary greatly even in individual tissues ; and, at a 

 rough estimate, examples may occur as large as a thousandth, whilst 

 others are so small as the four or five-thousandth of an inch. They 

 should be measured, if possible, in their natural state ; when injected, 

 their size is slightly increased ; but when dried, they diminish so con- 

 siderably, that in some specimens vessels imperfectly filled with injec- 

 tion have been known to shrink from the three to the twenty-thou- 

 sandth of an inch. 



We here digress for the purpose of saying a word or two on a 

 popular belief; that the precious organ of vision is liable to injury 

 from using the microscope. We have taken some trouble to look 

 through the records of a large hospital devoted to diseases of the eye, 

 and have likewise made many inquiries amongst microscopists, and 

 can come to no other conclusion, than that such a belief is without 

 foundation ; the eye, on the contrary, which may be most used in 

 making microscopic examinations, becomes the strongest and best. 



