42 ■ STUDIES IN BIOLOGY. 



Paris in that longer time may "be taken in injecting, as it does not 

 set and obstruct the vessels. By putting the specimens injected 

 with gum-arabic injection into spirits of wine the gum is precipitated 

 in the vessels. 



I have used chiefly the second method — that with plaster-of- Paris. 

 The most convenient point for injecting the arterial system is the 

 ventricle. A fine nozzle inserted into the ventricle can readily be 

 pushed forward into the neck between the ventricle and the aortic 

 bulb, and there firmly tied. The venous system can be injected 

 from the oblique vein or from the foot. One of the best methods of 

 preparing the mussels for injection is to remove the shell carefully 

 above the pericardium, then to open the pericardium and the 

 ventricle. In this condition the mussels should be placed back 

 downwards, and allowed to bleed. Successful injections were often 

 made in the case of specimens put for a day .or two in weak spirits of 

 wine. 



Sections (Figs. 41 to 44). 



Another method of examining the mussels was by cutting vertical 

 sections. By cutting thick sections with a sharp razor, holding the 

 mussel firmly on a block with the fingers, many points can be made 

 out ; but for sections that are to be examined under the microscope 

 special preparation of the tissue is necessary. The mussel, having been 

 carefully removed from the shell, is hardened in absolute alcohol and 

 stained with borax carmine, then set in a mould with melted caocao 

 butter. Before surrounding it with caocao butter the mussel must be 

 soaked in creosote. The slides are prepared by coating them very 

 thinly with shellac, by which the sections are firmly attached to the 

 slide. The sections are made by the use of a microtome and razor, 

 and are arranged in order on the prepared slides. Then, by heating 

 over a water-bath, and flooding repeatedly with turpentine, all traces 

 of the butter are removed, while the sections themselves keep their 

 places on the slides. The sections are then mounted in the usual way 

 in Canada balsam. Of a very young M. edulis I cut a complete 

 series of sections, there being in all 218 sections, and, of these, four 

 from different regions are shown enlarged in Figs. 41, 42, 43, and 44. 

 By this treatment many points concerning the histology of the tissues 

 are clearly shown ; but I have not had the time requisite for studying 

 the histology. In explanation of the plate of sections : — 



Fig. 41. [The side shown is the anterior side of the sections.] 

 This is a vertical section, a little behind the anterior end of the ante- 

 rior retractors, in the region of the labial palps and the gullet. The 



