624 



VERTBBRATBD ANIMALS. 



Fig. 330. 



438. In no part of the circulatiag apparatus, however, does 



the disposition of the capillaries 

 present more points of interest, than 

 it does in the Respiratory organs. 

 In Fishes, the respiratory surface is 

 formed by an outward extension 

 into fringes of gills, each of which 

 consists 'of an arch with straight 

 lamiuse hanging down from it ;'and 

 everyone of these laminae (Fig. 330) 

 is famished with a double row of 

 leaflets, which is most minutely 

 supplied with bloodvessels, their 

 network (as seen at a) being so close, 

 that its meshes (indicated by the 

 dots in the figure) cover less space 

 than the vessels themselves. The 

 gills of Fish are not ciliated on their 

 surface like those of Mollusks and 

 of the larva of the Water-newt; the 

 necessity for such a mode of I'enew- 

 ing the fluid in contact with them, 

 Two branchial processes of iheftHr^ttebemg supcrsedcd by the muscular 



Ed, showing the branchial laraellas:— a. apparatus with wllicll the gill-cham- 



_, ,___,j - ^^ furnished. But in Reptiles, 



the respiratory surface is formed by 

 the walls of an internal cavitj', that 

 of the lungs: these organs, however, are constructed on apian 

 very different from that which they present in higher Vertebrata, 

 the great extension of surface which is effected in the latter by 

 the minute subdivision of the cavity, not being here necessary. 

 In the Frog (for example) the cavitj- of each lung is undivided ; 



its walls, -which are thin and 



membranous at the lower part, 



pf "jj ^ |L . 'jil^§£y{ there present a simple smooth ex- 



^ M™lft& Wkf'S ^&1 »^>r P^^^se ; and it is only at the upper 



pai't, where the extensions of the 

 tracheal cartilage form a network 

 over the interior, that its surface 

 is depressed into sacculi, whose 

 lining is crowded with blood- 

 vessels (Fig. 331). In this man- 

 ner, a set of air-cells is formed 

 in the thickness of the upper 

 wall of the lung, which commu- 

 nicates with the general cavity, 



iilenor bf upper part of i"»j/ o/i^rof/. , . o i 



and very much increases the sur- 

 face over which the blood comes into relation with the air; but 

 each air-cell has a capillary network of its own, which lies on 



porlion of one of these processes enlarged, Up« 

 showing the capillary network of the la- 

 mellae. 



