288 



RESPIRATION. 



of the body receives blood directly from the 

 cardiac ventricle. The carotids, the homo- 

 logues of the subclavian, the hyo-opercular 

 and orbito- nasal arteries proceed from the 

 abdominal aorta at the point of confluence of 

 the branchial veins. In these vessels, there- 

 fore, the blood is arterialised, while its move- 

 ment is impulsatile or venous. The pro- 

 pulsive agents, under the form of diminutive 

 lymphatic hearts, which Dr. M. Hall*, M. 

 Fohmanf, and J. M tiller f have shown to 

 guard the several points of communication 

 between the absorbent and venous systems, 

 probably renders as great assistance in cir- 

 culating the contents of the latter as those of 

 the former orders of vessels. In the white- 

 bait (Clupea alba of Yarrell) these micro- 

 scopic hearts can be most perfectly observed. 

 From a consideration of the preceding traits 

 distinctive of the circulation of fishes, it will 

 be afterwards shown that the laws of aquatic 

 respiration are destined to receive new eluci- 

 dation. 



Minute Circulation of the Sranchice. The 

 branchial arteries proceed on either side, 

 symmetrically, from the aorta, and travel 

 through a groove along the convex border of 

 the branchial arches, the veins lying to the 

 outside of the artery, that is, next to the pec- 

 tinated fringes of the gills. The cartilage 

 system of the arches and that of the penknife- 

 shaped processes (bearing the ultimate bron- 

 chial leaflets) are quite distinct and uncon- 

 nected save by fibrous structure. It is in the in- 

 tervals between these solid parts that the trunks 

 of the vessels are disposed. As the pectinated 

 processes (a, Jig. 235. J arise, when biserial, 

 alternately from the arch, the arterial branches 

 leading to them observe a similar arrange- 

 ment. The arteries carrying venous blood 

 invariably run along the thick border or outer 

 margin of each process, the vein occupying 

 the innermost edge. A branchial process, 

 attached at right angles to the convex border 

 of the arch, resembles a penknife, the back 

 of which corresponds with the thick and 

 outermost borders, and the edge of the blade 

 with the thin or acute side of the process. A 

 string carried from base to point would mark 

 the position of the branchial artery convey- 

 ing venous blood : brought back along the 

 acute edge, it would denote the line of the 

 branchial vein bearing arterial blood. In 

 the marsupio-branchii (as the myxine, lam- 

 prey, &c.), in which the gills are fixed and 

 inopercuJate, this is also the virtual arrange- 

 ment of the secondary trunks. In the lopho- 

 branchii (sea-horse, pipe-fish, &c.), in which 

 the branchiae are tutted, the disposition of 

 the minute vessels is not dissimilar. The 

 flat surfaces of the penknife-shaped process 

 are gorgeously festooned by foliaceous multi- 

 plications of the membrane (jig. 235, 6). In 

 the annexed illustration, one of these leaflets 

 has been shown. In length it does not coincide 



* Essay on the Circulation of the Blood, 

 f Sangader system der VVirbelthiere. 1827. 

 \ Elements of Physiology, bv Dr. Baly, 2nd ed. 

 Lond. 1839. 



with the width of the process ; the secondary 

 trunks, passing through the substance of the 



A. Two lamella or penknife-shaped processes of the 



branchial arch of the Cod-fish. ( Original.) 

 b, the vascular respiratory plica;, which extend 

 only three-fourths across the flat surface of the 

 process. They are seated on the internal longi- 

 tudinal half or edge of the penknife-shaped process. 



cartilage (b) at right angles with the primary 

 (a) are longer than the corresponding se- 

 condary veins on the opposite side of the 

 bladelet ; hence the one-sided position of the 



Fig. 236. 



Represents a transverse section of one of the penknife- 

 shaped processes (with a single leaf bearing the 

 ultimate respiratory capillaries on each side) ; the 

 web of vessels forms a single layer, and is covered 

 by a pavement epithelium. ( Original.) 

 c denotes the elastic chord which runs along the 



circumferential border of the leaflet. 



leaflet (c) bearing the capillary network. These 

 membranous processes, by which the active 

 breathing surface is so extensively multiplied, 



