• Respiration in Invei'tehr ate Animals. 185 



In the dorsal vessel, from the extreme tail to the extreme head, 

 tlic blood moves forwards, in the ventral backwards : in all 

 bj'anches whether integumentary or visceral, from the latter into 

 the former. The dorsal vessel is afferent only with respect to 

 those parts which are situated anteriorly to the oesophageal ring; 

 the ventral is efferent only to the same parts — it is distributive 

 to all others. 



The preceding account of the course of the blood in the My- 

 riapoda embraces conclusions suggested by careful study and 

 numerous observations; it is recommended by its meehanical 

 simplicity; it entangles the physiologist in no hydraulic con- 

 tradictions. 



That of the Insect (fig. 2) is regulated in exact conformity 

 to the myriapodal type. The dorsal vessel (a) in the Insect 

 exhibits signs of concentration ; it is only the abdominal portion 

 that is multiplied into chambers ; through the thorax the vessel 

 is continued in form of a simple tube. The auricular orifice ®f 

 these chambers is furnished with a more perfect valvular appa- 

 ratus. The same observations apply to the so-called systemic 

 arteries of Insects as to those of Myriapoda. To the presence 

 of these vessels in this class, however, no allusion is made by Mr. 

 Newport*. If they exist in the Myriapod, they must also exist 

 in the adult Insect. 



The supra-spinal trunk (fig. 2, b) in Insects, as in Myriapods,- 

 is the great centre of the ventral circulation. It receives all its 

 blood from the dorsal vessel by means of the anterior branches 

 into which the latter divides; it supplies chiefly the external 

 structures and the nervous chords. Either from the descending 

 aortic branches or from the anterior part of the supraspinal artery,- 

 there proceed backwards along the ventral aspect of the viscera,, 

 one or more secondary trunks which correspond with those which 

 in the Annelids are exclusively distributed over the parietes of the 

 alimentary canal ; in these, the direction of the blood corresponds 

 with that in the great ventral artery ; like the latter, they are 

 afferent and distributive. All the blood, thus, by means of these 

 trunks, entering the viscera from below, reaches again the dorsal 

 vessel, conducted by the so-called " systemic arteries." In Insects 

 the vessels connected with the abdominal circulation are more 

 perfect, mechanically, as conduits, than those traversing the exte- 

 rior structures of the body. This explains the difference of 

 structure which exists between the systemic arteries (sic) and the 

 loose membranoiis channels (fig. 2, ff,f, e) opening into the auricles- 

 of the heart. The former belong to a distinct segment (the vis- 

 ceral) of the circulation, the latter to the integumentary or peri- 



* See article Insecta, Cyclop. Anat. and Phys. 



