CIRCULATION OF THE BLOOD. 25 



there is not a well defined line of demarcation between the 

 (Esophagus and stomach, which is simple and various in form. 

 The intestines are short and without a ccecal appendix ; the large 

 intestine differs little from the small, and terminates in a cloaca. 

 The liver is generally voluminous, and there is a gall bladder, 

 pancreas, and spleen. 



19. The blood of these animals is red, and its globules are 

 elliptical. Their volume is much greater than in mammals and 

 birds, and the number of them is considerably less. The dispo- 

 sition of the circulatory apparatus varies; but there- is always a 

 direct communication between the vascular system of red 

 (arterial) blood, and the vascular system of black (venous) blood, 

 so that these two liquids mingle with each other, and the organs 

 receive only blood, imperfectly arterialised by the act of respiraton. 

 The heart is almost always composed of two auricles, (or/, og,) 

 which empty into a single ventricle, (r.) 

 Therefore, the arterial blood coming from 

 the lungs and received in the left auricle, 

 (or/.)> and the venous blood coming from *~^ 

 different parts of the body into the right 



auricle, mingle in a common ventricle, (r.) | 'fffl^.^ Fig 1. 



One part of this mixture returns by the 

 aorta to the different organs it is des- 

 tined to nourish, and another goes to 

 the lungs, through the pulmonary arte- 

 ries (ay;,) which rise immediately from 

 the common ventricle or from the aorta itself.* It appears that 

 in crocodiles, the heart is the same in form as it is in birds and 

 mammals, and a partition separates the right ventricle from the 

 left : therefore the arterial does not mingle with the venous blood ; 

 but there is a particular arrangement of the arteries which effects 

 this mixture at a short distance from the heart, and the vessels 



* Explanation of the Figure. Heart and principal vessels of a tortoise, 

 c, ventricle, od, right auricle, which receives the blood from the great 

 venous trunk, (re,) and pours ft into the ventricle, (v) ; Off, left auricle, 

 which receives the arterial blood coming from the lungs by the pulmonary 

 veins, (vp) and also pours it into the ventricle; ag,ad, the two aortas which 

 arise from the single ventricle, arid which, after being carried backwards, 

 unite to form the vertical aorta, (av) ; oc, branch of the right aorta which 

 furnishes the carotid, brachial arteries, &c. ap, ap t the two pulmonary 

 arteries, the common trunk of which arises from the ventricle along side of 

 the aortas. The arrows indicate the course of the blood. 



19. What is the colour of the blood in Reptiles? How does their circu- 

 lation differ from that of mammals ? What is the course of the circulation 

 in Reptiles? 

 2 



