33 



HEART. 



HEART. 



31 



The simplest kind of a heart, forming a defined cavity, 13 found in 

 the larger Decapoda, and some other Crustacea. The annexed sketch 

 represents the heart and large arteries of the lobster. There is a 

 single cavity or ventricle (a), into which the arterial blood flows from 

 the gills by vessels which unite into two trunks, whose ori6ces 

 (6, b) are protected by valves. Six main arteries proceed from the 

 heart ; the three anterior (c, c, c) go to the head ; two from its inferior 

 part to the liver ; and the largest of all (d) from its posterior part. 

 This last gives off a superficial and 

 a deep artery to the tail, and then 

 curving forwards under the ster- 

 num, gives branches to the feet and 

 deep arteries to the head. From 

 these the blood returns by veins 

 into a number of venous sinuses 

 which lie at the sides by the articu- 

 lations of the legs with the chest, 

 and all communicate together. 

 From them branchial arteries pro- 

 ceed, which run on the outer edge 

 of the gills, and pass through capil- 

 lary vessels in them, terminating 

 in branchial veins which unite into 

 two trunks that open into the heart. 



In most of the Afollusca the 

 blood flowing through the bran- 

 chial veins, instead of being poured 

 directly into the ventrical, is re- 

 ceived first into an auricle, present- 

 ing the first instance of a heart 

 with more than one cavity. In 

 most of the Gasteropoda and 

 Plempoda the auricle is single ; in 

 the bivalves the auricle is mostly 

 double. In the latter division, 

 the blood, collected from the systemic veins into one venous 

 reservoir, before going to the gills passes by numerous branches into 

 a spongy tissue consisting of lacuna;, which Bojanus calls a lung, 

 and others a kidney. From this a few branches pass at once into the 

 auricles, but the greater number go as branchial arteries to the gills. 

 In the brachiopods the systemic ventricle is also double, each cavity 

 giving rise to an aorta. In these cases, although the auricle is double, 

 it is not to be regarded as representing the two auricles of warm- 

 blooded animals, of which one receives the blood from the system, 

 the other from the lungs, since here the two auricles have the same 

 function, and there are two for the sake of convenience. In the oyster 

 they are united into one cavity. But the most singular form of heart 

 in this class, and one of much interest as presenting all the cavities 

 separated, which in higher classes are united in one body, is that of 

 the cuttle-fish. The blood passes in it from a systemic or central ven- 

 tricle (a), through the aorta superior (6), and inferior (c), and is thence 



d 



dbtrlboied throughout the body, whence it is collected into six venso 

 cavnc C</, </, if, '', </, d), which open into two branchial auricles (, ), 

 ;<:ting into two branchial ventricles (/, /> by which it is forced 

 ii branchial arteries (;/, g, g). From the extremities of these it 

 in received into branchial veins, of which that of the left side (A) is 

 here shown, which open into the two systemic auricles (', i), conduct- 

 ing agnin into the systemic ventricle (a). Here then wo have the 

 HIST. DIV. VOL. III. 



a 



f 



first appearance of a separate heart for the respiratory circulation, 

 aud the elements of all the compound forms which we have now 

 to notice. 



Among vertebrate animals the simplest form of heart is found in Fish. 

 They have a single auricle (a, a), which receives the blood from tho 

 trunks of the veins of the whole body (6, ft), and communicates with 

 a single ventricle (c), 



which forces the 



blood into an arte- 

 rial trunk with a con- 

 tractile bulb (d). 

 From this trunk all 

 the branchial arteries 

 (e, e) arise, and passing 

 on each side in arches 

 to the gills, divide 

 there into capillary 

 branches. The blood 

 thus aerated passes 

 on, and the arches 

 again unite into a 

 common trunk (/), 

 the true aorta, which 

 runs along the under 



surface of the spine, b 



and sends the blood 

 to all parts of the 

 body. From these it 

 collects again into the 



systemic veins (6, 6), It 



by which it is carried 



back to the auricle. 



The same type of 



formation is found in 



the reptiles which 



have gills, as in the 



Proteida in their 



adult state, and in the lan'aj of many which at that period also 



breathe by gills. 



In Fish all the blood is subjected to the respiratory process before 

 it passes to the body : in the Reptiles, which breath in air, and have 

 therefore a much more complete exposure of the blood to it than fish, 

 who breathe only the air that is dissolved in the water, only a part is 

 exposed before entering the general circulation, but the modes in 

 which this is effected vary greatly. The annexed sketch represents 

 the circulatory system of the Frog in its perfect stato. It has a single 

 ventricle (a), from which arises a single aortic trunk (6), from which 

 proceed the two pulmonary arteries (c, c), conveying that part of the 

 blood which is to undergo respiration to the lungs (d, d), whence it is 

 returned by pul- 

 monary veins (e, e). 

 From the same 

 aortic trunk four 

 other arteries arise, 

 two of which, 

 forming arches, 

 unite to form the 

 dorsal aorta, while 

 the two uppermost 

 are distributed to 

 the head and upper 

 extremities. From 

 the terminations of 

 these arteries in all 

 parts of the body 

 the blood is col- 

 lected into large 

 venous trunks, 

 which open into 

 the double auricle 

 (/, /), from which 

 the aerated blood 

 from the lungs and 



the impure blood from the system pass separately into the common 

 ventricle, in which they are intimately mixed. In other reptiles 

 the ventricle is more or less completely divided by a septum, 

 which in the Crocodilua lucius separates it into two distinct 

 cavities, one connected with a pulmonary, the other with a systemic 

 aorta ; in some others the septum is nearly complete, and the mixture 

 of the two kinds of blood is supposed to be prevented by a valve ; 

 but in the rest of the order the septum is so small that the blood 

 must necessarily be mixed. In all of this order however the abdominal 

 aorta, which runs along the spine, is formed, as in the frog, of two 

 arches, and in those which have separate ventricles a branch proceeds 

 from each to form it, so that while the head and upper extremities 

 are supplied with pure arterial blood from the left side of the heart, 

 th lower portion of the body receives a mixed blood from tho left 

 side and the right. 



