IQ2 HOW TO WORK 



by accident. The most scrupulous care must always be observed in 

 microscopical examination, and any foreign particles which may have 

 accidentally come into contact with the preparation must be removed 

 before it is mounted. The proceeding to be followed to remove the 

 foreign matter, will depend much upon its nature. Mere dusting 

 with a camel's hair brush, washing in a stream of water, or picking 

 out the object with needles, are simple plans which are often efficient 

 in a general way, but in some cases other processes are required. 



2*38. Errors of Observation. The important thing is to avoid 

 making erroneous observations. One is liable, not only to draw 

 false conclusions from observations, but the observations themselves 

 are frequently erroneous. I propose therefore, to direct the student's 

 attention to a few of what appear to me frequent sources of difficulty 

 and doubt even to the most experienced. 



Of the Commencement and Termination of Tubes. The modes of 

 commencement or termination of certain vessels or tubes have long 

 been sources of dispute among observers. There are not a few 

 instances where positive statements have been made that certain 

 tubes commenced by ccecal or blind extremities ; while contradictions 

 equally positive have been advanced by others, who have affirmed 

 that the very same tubes commenced as a network, and presented no 

 blind extremities whatever. It would be supposed by many that this 

 point might be determined beyond all doubt by injecting the tubes 

 with some coloured material. But this is not so. Injection will 

 frequeatly run up to a particular point in the minute vessels, while 

 no force which could be employed could drive it further onwards. 

 Here, therefore, it accumulates, and often to a very considerable 

 extent j the portion of the tube above the constriction being con- 

 siderably dilated by the pressure. Under these circumstances it may 

 be impossible to trace the further continuity of the vessel, owing to the 

 extreme transparency and delicate nature of the tissue of which its 

 walls are composed. Indeed, these may be quite invisible in an 

 unprepared specimen. The observer is thus led into the error of 

 supposing that such tubes terminate in blind extremities, whereas 

 they may really form a network with large meshes, or they may be con- 

 tinuous with other structures beyond. In fact that which was taken 

 for the termination or commencement of the tube may really be nothing 

 more than a bulging in a central part of its course. In many thin 

 sections of the kidney an appearance as if the tubes terminated in 

 free blind extremities is produced in consequence of the convolutions 

 lying in such a position that the recurved portion is immediately 

 beneath the most superficial part of the tube. From a mere examina- 

 tion of the specimen it would be impossible for any one to say that 



