IO4 HOW TO WORK 



In this case the injecting fluid passes into lacunae or spaces and fills 

 the venous system, but as has been shown by Mr. Robertson, of 

 Oxford, the arteries are not injected by this method (On the Organs 

 of Circulation of the Roman Snail, Helix Pomatia. Annals and 

 Magazine of Natural History, January, 1867). 



192. Mr. Robertson's Plan of Injecting the Snail. This skilful 

 anatomist recommends another plan of proceeding. The snails are 

 to be killed by drowning them in a jar quite filled with cold water, 

 the mouth being closed with a piece of plate glass. 



The vascular system is to be injected from the ventricle of the 

 heart with size and carmine. The heart of the snail is easily found. 

 It is enclosed in a sac which is situated at the posterior extremity of 

 the pulmonary chamber on the left side. The position of the organs 

 of the snail has been fully described by Dr. Lawson, in a paper 

 published in the Mic. Journal for January, 1863. The injection 

 introduced into the heart passes right round the body and returns to 

 the pulmonary chamber. By this plan the arterial branches may 

 be traced into the foot and to many other parts which were con- 

 sidered to be destitute of arteries. Mr. Robertson has arrived at 

 the conclusion that in snails there exists a closed capillary system 

 communicating directly with the arteries on the one hand and the veins 

 on the other as in the higher animals ; and he has completely failed to 

 demonstrate the existence of any direct communication between the 

 spaces or lacunae in the various tissues and the vascular system. It 

 is probable that in the mollusca the capillaries are arranged as in the 

 higher animals, but they are wider, and their walls being so very thin 

 it requires great skill to inject them without extravasation. 



193. injecting Fishes. The vessels of fishes are exceedingly 

 tender, and great caution is required in filling them. It is often 

 difficult or quite impossible to tie the pipe in the vessel of a small 

 fish. If we attempt to inject from the heart, the injection passes to 

 the gills, but it is seldom that it runs through these and penetrates 

 the systemic vessels. It is usual therefore to proceed thus, the tail 

 of the fish is to be cut off, and the pipe introduced into the divided 

 vessel which lies immediately beneath the spinal column. In this 

 simple manner beautiful injections of a fish may sometimes be made. 

 In small fishes in which the vessels are too delicate to be tied, a 

 good injection may be made by simply placing the pipe in the vessel. 

 As the fluid is so cheap, a considerable loss is of no importance. 



194. Of Preparing Portions of Injected Preparations for Micro- 

 scopical Examination. Preparations made by injecting colouring 

 matters suspended in water or gelatine may be mounted in various 

 preservative fluids, or dried and placed in balsam. When thin 



