984 



PISCES. 



near to large veins covered by strong cartilages, 

 winch resemble our clavicles, and which there- 

 fore may be called subclavian veins. Towards 

 these all the lymphatic vessels of the body are 

 directed, the lymphatics of the kidneys and 

 organs of generation with those of the tail and 

 inferior parts ascending, those of the flesh and 

 side fins of the trunk of the body running in- 

 wards, and those of the superior parts and of 

 the brain, organs of the senses, heart, and gills, 

 descending. 



The branches of the lymphatic vessels form 

 larger angles where they terminate in their 

 trunks than are found in the veins, and the 

 smaller branches are connected by transverse 

 canals. The large lymphatics of the muscular 

 organs, near their joining with the lacteals, are 

 collected together in the most simple manner, 

 without forming such intricate plexuses as are 

 found in the course and near to the termination 

 of the lacteal vessels : the lymph from the 

 head and thorax in particular is conveyed 

 chiefly by a single trunk, which receives large 

 lateral branches from the adjacent parts. At last 

 a single vessel on each side of the fish, and in 

 which there is no dilatation or large receptacle, 

 receives all the chyle and lymph, and terminates 

 in the subclavian vein of the corresponding 

 side very near its junction with the internal 

 jugular vein, or nearly in the angle which these 

 two vessels form by their joining. The blood is 

 prevented from getting into these two vessels 

 by a pair of valves placed at the termination of 

 each . 



In the osseous Fishes the course of the 

 lymphatics has been traced both by Monro and 

 Hewson,* principally in the Gudidie and the 

 Salmon : their general arrangement has been 

 described above. 



Four lymphatic vessels which terminate in 

 the subclavian receptacles chiefly merit attention. 

 The first conveys the lymph from the ventral pa- 

 rietes, from the ventral and pectoral fins, and from 

 the heart. The second runs up the side of the 

 fish parallel to the great mucous duct, and brings 

 the lymph from the principal muscles of the 

 tail and body. The third is deep-seated and 

 conveys the lymph from the spine, spinal me- 

 dulla, and upper part of the head, while the 

 fourth lymphatic vessel, or rather plexus of ves- 

 sels, brings the lymph from the brain and organs 

 of the senses, and from the mouth, jaws, and 

 gills. 



The two receptacles into which all these ves- 

 sels open communicate freely with each other 

 by wide canals, which pass chiefly behind the 

 heart and oesophagus, and each ultimately 

 empties itself into the upper end of its corre- 

 sponding vena cava inferior, contiguous to the 

 termination of the internal jugular vein, the 

 communications between the lymphatic and 

 venous systems being guarded by valves. 



It will be seen from the above description that 

 the lymphatic system of Fishes offers several 

 remarkable peculiarities when compared with 

 what is met with among the higher Vertebrata, 

 amongst which may be noticed the total want 



* Phil. Trans, for 1769. 



of lymphatic glands, and the absence of valves 

 in the absorbent trunks. Owing to this latter 

 circumstance, nothing is more easy than to in- 

 ject them from trunk to branch, and thus dis- 

 play their minute ramifications with the greatest 

 ease. In this manner Mr. Hewson detected in 

 the Cod a beautiful net-work of lymphatic 

 vessels situated between the muscular and vil- 

 lous coats of the intestine, something analogous 

 to what exists in the Turtle ; but in Fish it is 

 more evident that there can be no deception, 

 seeing that the injection is contained in cylin- 

 drical vessels, not diffused in cells, as is the 

 case in the reptile. If mercury be injected 

 into this net-work, it spreads over the intestine ; 

 and if the intestine be inverted and slightly 

 pressed, it can readily be seen to pass into the 

 villi of the intestinal mucous membrane. In 

 the stomach, however, the absorbents have a 

 different arrangement. When minutely in- 

 jected with mercury, they are seen to pass 

 through the external coats, dividing into smaller 

 and smaller branches, without any appearance 

 of a net-work between the muscular and the 

 villous coat, so that the absorbent vessels of the 

 stomach manifestly exhibit a different arrange- 

 ment from those of the intestine. 



By adopting a similar mode of injecting from 

 trunk to branch, Dr. Monro succeeded in de- 

 monstrating numberless lymphatics in the brain, 

 eye, ear, and nose, in all of which organs their 

 existence had been previously doubted ; and, 

 also, in proving that their ramifications in 

 the rest of the body were far more extensive 

 than had been supposed. He, moreover, points 

 out another circumstance of considerable im- 

 portance in a physiological point of view, viz., 

 that on examining the minute branches of the 

 lymphatics, they are found to consist of an im- 

 mense number of anastomosing canals, many 

 of which enter the neighbouring lymphatics at 

 right angles instead of being directed towards 

 the heart, by which means a net-work is pro- 

 duced so very intricate that, when a small por- 

 tion only is examined, it is difficult or next to 

 impossible to ascertain what has been the 

 natural course of the lymph ; it is therefore 

 evident that, from the great number and unfa- 

 vourable direction of these canals, general pres- 

 sure cannot in this case be a chief cause of the 

 progressive motion of the lymph, but that each 

 vessel must contribute to its progress by a well- 

 regulated action. 



Another remarkable circumstance is stated 

 by Monro in connection with the lymphatic 

 vessels, namely, that in the Skate they open 

 by patulous orifices situated upon the back of 

 the Fish, of sufficient size to allow not only air, 

 but water, milk, quicksilver, and even oil of 

 turpentine coloured with vermilion, to be dis- 

 charged upon the surface of the skin, even 

 when the force employed in injecting these 

 fluids was very slight, and no extravasation was 

 produced into the cellular tissue either under 

 the skin or in the muscular interstices. The 

 function attributed to these open vessels by 

 Monro is, however, even if they exist, quite 

 hypothetical, namely, that they are for the 

 purpose of absorbing from the ocean the fluid 



