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551 



the access of atmospheric air to it, as it passes through 

 the capillary vessels of the lungs. These must evi- 

 dently be taken together, because, if the one of them 

 is by any means stopped, the other stops along with 

 it, or at least immediately after. 



These two functions, or rather the compound func- 

 tion of which they are the t\vo most essential parts, 

 may be regarded as the indices of the quantity of life 

 the rapidity of habitual action in the animal, though 

 not of the momentary efforts that it may be capable 

 of making at long intervals. They are purely animal 

 operations, carried on in opposition to physical causes 

 from without, and not in any way by the influence of 

 such causes ; and therefore, the more vigorous that 

 the circulation and respiration of any animal are, the 

 better is the animal able to bear the variations of 

 climate and the vicissitudes of the weather. The 

 fact that reptiles are, in the great bulk both of their 

 species and their numbers, confined to the warm 

 lands and seas, and to those places of the lands which 

 have the highest temperature in their atmosphere, 

 might be taken as conclusive evidence that in them 

 the circulation and the breathing must be both very 

 feeble, otherwise they would not leave the animals so 

 much at the mercy of the elements. 



Besides the difference in the active parts, the heart 

 and the lungs, there are other evidences in the whole 

 reptile class. First, the whole quantity of the blood 

 is much smaller than in mammalia, or than in birds, 

 though in the majority of these it is less than in the 

 majority of the mammalia ; from which we would 

 infer that the voice and the sagacity of animals are 

 in proportion to their quantities of blood. The mus- 

 cular fibres of all reptiles are white, and they bleed 

 little or none when cut into, if the larger vessels are 

 avoided. They consist more of elastic membrane, 

 composed chiefly of gelatine; and it may perhaps be 

 presumed that this substance is more under the con- 

 troul of what we may call the electric influence than 

 fibrin, which is a product of blood. Secondly, the 

 change which the blood of reptiles undergoes in pass- 

 ing through the lungs is much less than in mammalia 

 and birds. In these there is a very marked difference 

 between the bright red arterial blood, which is sent 

 over all parts of the body for their nourishment and 

 repair, and the dark red venous blood, which comes 

 back to be renovated in the lungs; but if we examine 

 the blood from these two parts of the system in a 

 reptile, we can hardly tell the venous from the arterial 

 either by the colour or by any other tint. It should 

 seem indeed that there is some intercourse between 

 the solid part of a reptile and its blood, independently 

 of its circulation ; for when the animal continues for 

 a long time in a lethargic state, without activity or 

 food, the blood in all parts of the system becomes 

 paler in the colour, as if that portion of it upon which 

 the colouring matter depends were taken up by ab- 

 sorption. Toads, which have been long shut up in 

 the holes of walls, or have been "grown in" in trees, 

 are said to have little or no red colour in their blood. 

 This would lead us to conclude that the system of 

 the blood, which is the controlling system in mam- 

 malia and birds, is a controlled system in reptiles ; 

 and as the nervous system is always in accordance 

 with the system of the blood, that must be a con- 

 trolled system in reptiles as well as the other is. 



The form and structure of the heart vary a great 

 deal in the different orders of reptiles. In some 

 there is a rudiments! appearance of a double heart, 



though there is always some internal communication 

 by means of which the systematic and the pulmonary 

 circulations are more or less blended together, so that 

 a greater or smaller portion of the blood, which has 

 been returned to the heart by the systematic veins, 

 can be returned directly by the systematic arteries, 

 without having passed through the lungs. Whether 

 the communications between the cavities are capable 

 of being contracted or expanded, so as to Vary the 

 portion of circulation through the lungs, has not been 

 very clearly ascertained ; but it is highly probable 

 that these internal communications in the heart con- 

 tract when the animal is greatly wasted, and expand 

 as it relapses into a state of inaction. This does not 

 of course admit of direct experiment, but it is at 

 least highly probable. Most of the animals that have 

 the double circulation perfect and vigorous are read\ 

 for action on the instant ; but there is always an 

 effort of preparation required by a reptile ; and it is 

 highly probable that this preparation is not confined 

 to the external demonstrations, but that a part, and 

 that the most essential part, goes on within ; that in fact 

 it contracts these natural passages of the heart, and, 

 for the occasion, approximates to the character, and 

 thereby acquires a portion, though a momentary one, 

 of an animal with a perfect double circulation. In 

 this again we can learn the indefinite and shadowy 

 place of the reptile in nature, not of the earth or the 

 air on the one hand or of the water on the other, but 

 of the boundary between both. Some reptiles have 

 the heart far less perfect than this, and they are not 

 capable of even momentary displays of energy, but 

 pass their time in an unbroken dulness and repose. 

 It may not be amiss to name the differences in the 

 three orders, leaving out the serpents, as we have 

 done in the other parts. 



In the (Jhclonia the heart is broad, and it is nearly 

 semicircular in the turtles, and of a square form in the 

 tortoises ; in both there are two auricles, one system- 

 atic and the other pulmonary, but they open into 

 one ventricle ; this ventricle is divided into two parts 

 by a partition, but there is an opening in this par- 

 tition which, as we have said, probably varies in 

 capacity. 



In the Sauria the heart has also two auricles, and 

 as many of these reptiles are active, and most of 

 them can be worked into energy upon occasion, wo 

 might expect that the heart should be more complex 

 than it is among the Chelonla. The crocodiles are 

 hence the most typical, the most powerful, and the 

 most celebrated of this order, and therefore we shall 

 confine our limits to the heart as it appears in them. 

 It is evidently formed for vigorous action, although 

 that action may be momentary ; and the ground upon 

 which we come to this conclusion is very certain- 

 namely, tho great strength of the organ, and tho 

 power of the muscular fibres with which it is supplied. 

 The auricles are much smaller in proportion than in 

 the heart of the Cliclonia, but they are far thicker 

 and more muscular in the walls. The ventricle is also 

 very strong, and of an oval form ; it is divided into 

 three compartments, which communicate with each 

 other by numerous orifices, all apparently capable of 

 expansion and contraction. The right compartment 

 sends the blood to the hind part of the body, and 

 partly, also through the orifices, into the middle part ; 

 the left, part sends it towards the fore part of the 

 bod\ r , and also in part to tho middle compartment, 

 which is the pulmonary portion of the ventricle, and 



