220 MisceUaneous. 



and througli this a coloured fluid be introduced into the circulation, 

 the heart drives this liquid to the extremities of the finest arteries. 

 Arrived at the last ramifications, the blood certainly slackens in its pro- 

 gress whilst passing through the capillary network. This network, 

 which has not yet been pointed out in the Articulata, exists under 

 the teguments and between the various layers of muscles in the con- 

 nective tissue ; it consists of distinctly circumscribed canals lined 

 with a thin epithelium. Thence the blood is received by the venous 

 canals. 



These are only canals and sometimes sinuses and not tubes ; but 

 they are always lined with a membrane, which it is frequently not 

 difficult to detach from the surrounding tissues. There are no val- 

 vules to prevent the return of the blood. Nevertheless the blood 

 always follows a determinate course, and in fact the disposition of the 

 muscles favours the passage of the fluid in one direction, but presents 

 an obstacle, sometimes absolutely insuraiountable, to its passage in 

 the other direction. 



In injecting a coloured fluid into the abdominal cavity, it is seen to 

 penetrate readily into most of the spaces occupied by the venous 

 blood, whilst it does not enter the venous canals of the feet or other 

 ajipendages. A resistance is here presented which is not overcome by 

 considerable pressure ; the flexor and extensor muscles of the legs 

 approaching at their points of attachment close the passage from 

 within outwards. It will be readily understood, in fact, that without 

 some particular arrangement, the blood, existing in abundance in the 

 thoracic cavity, would always have a tendency to fall back in the 

 venous canals of the extremities. 



The legs are more or less exposed to be broken in the Arachnida. 

 These fractures always take place at the articulations, and the ani- 

 mal does not suffer much from them. A little drop of blood may 

 form at the extremity of the broken limb, but the contraction of the 

 muscles closes the artery, and the venous canal is so disposed that 

 the normal course of the blood is never interrupted for an instant. 

 At the extremity of each joint of the legs the large venous canal is 

 curved back so as to take up all the blood coming from the lesser 

 canals. This arrangement completely explains why when half the 

 leg of a Scorpion or Spider is torn off^ the blood escapes from the 

 detached portion, and not from that which remains attached to the 

 body. 



The venous blood collected from various parts of the body arrives 

 at last in canals situated at the sides and lower part of the abdomen, 

 whence it passes to the respiratory organs. The principal venous 

 canals are lodged in the interstices of the segments, and are formed 

 by a fold of the internal membrane. In the Scorpions, the venous 

 canals of the caudiform portion of the abdomen and those of the feet 

 consist of gutters which show themselves externally as so many 

 ridges. 



In the Arachnida, nearly as in the Crustacea, the blood returns to 

 the heart from the respiratory organs by means of vessels passing up 

 along the sides of the abdomen and opening into the pericardiac 

 caTidty. This is effected in the following manner. 



