ANIMAL STRUCTURE. 345 



vegetable life. Bichat terms it organic life and relative life. In organ- 

 ised beings, the way in which nature works out her most secret pro- 

 cesses is by far too minute for observation by unassisted vision ; even 

 with the aid of the improved microscope, only a small portion has, up 

 to this time, been revealed to us. To point out in detail the disco- 

 veries made through the employment of this instrument, as regards 

 physiology, would be to give a history of modern biological science j 

 for there is no department in this study which is not more or less 

 grounded upon the facts and teachings of the microscope. 



To the casual observer the brain and nerves appear to be composed 

 of fibres. The microscope,* however, reveals to us, as was first pointed 

 out by Ehrenberg, that these supposed fibres do not exist, or rather, 

 that they all consist of numerous tubes, the walls of which are distinct, 

 and contain a fluid which may be seen to flow from their broken ex- 

 tremities on pressure. In looking at a muscle, it appears to be made 

 up of fine longitudinal fibres only. The microscope tells us that each 

 of these supposed fine fibres is composed of numerous smaller ones, 

 and that these are crossed by lines which have received the name of 

 transverse strite ; that muscular contraction, the cause of motion in 

 animals, is produced by the relaxation or approximation of these trans- 

 verse striae. 



The microscope has shown us that a distinct network of vessels 

 lies between the arteries and veins, partaking of the properties of nei- 

 ther, and possessed of others peculiar to themselves. These have been 

 denominated intermediary vessels by Berres, and serve to connect the 

 arterial with the venous system. 



On regarding with the naked eye the different glands in which the 

 secretions are formed, how complex they appear, how various in con- 

 formation ! The microscope teaches us that they are all formed on 

 one type ; that the ultimate element of every gland is a simple saccu- 

 lated membrane, to which the blood-vessels have access ; and that all 

 glands are formed from the greater or less number, or different arrange- 

 ment only of the primary structure. 



Our notions respecting the skin were vague until the microscope 

 discovered its real anatomy, and showed us the existence and relations 

 of the papillae, of the sudorific organs and their ducts, the iuhalent 

 muscular apparatus, and so on. All our knowledge of epidermic struc- 

 tures, such as hair, horn, feather, &c., the real structure of cartilage, 

 bone, tooth, tendon, cellular tissue, and, in a word, of all the solid tex- 

 tures, has been revealed to us by the same agency ; so that it may be 

 truly said, that all our real knowledge of structural anatomy, and all 



