OF MYRIAPODA AND MACROUROUS ARACHNIDA. 
297 
The orifices in the common cavity of the branchiae (fig 1 . 31. fy), which communicate 
with the interior of the lamellae, are largest in the anterior and smallest in the poste- 
rior ones. The cavities of all the branchiae communicate directly with each other by 
a short narrow passage, so that the whole on one side of the abdomen forms one 
common cavity or lung-like organ, like the large tracheal vesicles in the abdomen of 
perfect insects, and thus ensures an uniformity of function at each act of inspiration 
throughout the whole body. 
Nutrition of the Branchiae. — Besides the vessels already described, the branchiae 
are supplied with arterial branches apparently for the nourishment of their tissues. 
The first arterial vessel is derived from the supra-spinal artery while passing over the 
vena cava. This vessel ( p ) accompanies the nerve (q) that is passing backwards to 
the fourth abdominal segment, behind the first branchia, to which it is distributed 
after supplying the trunk of the nerve in its course. The other yessels (u) are de- 
rived from branches of the systemic arteries. Treviranus* long ago described vessels 
in the Scorpion distributed on the branchiae ; but he acknowledged, in his description 
of similar vessels in the Spiders, that he was unable to determine whether they are 
arterial or venous. He seems to have thought that they carry back the blood to the 
heart. But this is not the case. They are derived from the great systemic arteries, 
along with which I have traced them upwards to their origin in each chamber. These 
then are the structures of the pulmonary organs in the Scorpions, a system as perfect 
as in any of the Articulata, and which will lead us clearly to understand the course 
of the circulation in these animals. 
Course of the Circulation. — The blood received by the veins from the branchiae is 
conveyed to the heart round the sides of the segments receiving accessions from other 
vessels in the segments in its course, and enters the heart at the posterior part of each 
chamber on its dorsal surface through the orifices of Straus. The auriculo-ventri- 
cular cavity, dilated by the influx of blood, begins first to contract by the action of 
the circular fibres at the posterior part of each chamber. By this contraction part 
of the blood is at once propelled laterally through the systemic arteries to the inte- 
rior and sides of the body ; while the remaining and chief portion is forced onwards 
through the valves and body of the chamber by the successive contraction of the cir- 
cular fibres into the next chamber. A fresh accession of blood enters the heart at the 
auricular orifices in the short interval of time that elapses between the contractile 
actions of the two chambers, which interval is probably occasioned by the reaction of 
the lateral muscular appendages of the organ. These contractions, commencing in 
the principal chamber in the sixth abdominal segment, are carried gradually onwards 
through the whole of the succeeding segments ; so that ere a third chamber has 
contracted the first is again filled and ready to be emptied, thus occasioning by their 
alternate movements those pulsatory motions observed in all instances in which the 
heart is formed of a longitudinal series of chambers and valves, motions which are 
* Vermischte Schriften, Anatomischen und Physiologischen Inhalts, vol. i. Gottingen, 1S16. 
