COMPARATIVE PHYSIOLOGY OF THE CIRCULATION. 215 



according to other observers, in some animals it is more like the thyroid. Its 

 functions are entirely unknown. 



Coccygeal and Carotid Glands. The former, which lies on the tip of the 

 coccyx, is composed, to a large extent, of plexuses of small more or less cavernous 

 arteries, supported and enclosed by septa and a capsule of connective-tissue 

 (Luschka). Between these lie polyhedral granular cells arranged in net- works. 

 The carotid gland has a similar structure (p. 124). Their functions are quite 

 unknown. Perhaps both organs may be regarded as the remains of embryonal 

 blood-vessels (Arnold). 



104. Comparative. 



The heart in fishes as well as in the larvse of amphibians with gills, is a simple 

 venous heart consisting of an auricle and a ventricle. The ventricle propels the 

 blood to the gills where it is oxygenated (arterialised) ; thence it passes into the aorta 

 to be distributed to all parts of the body, and returns through the capillaries of the 

 body and the veins to the heart. The amphibians (frogs) have two auricles and 

 one ventricle. From the latter there proceeds one vessel which gives off the pul- 

 monary arteries, and as the aorta supplies the rest of the body with blood, the 

 veins of the systemic circulation carry their blood to the right auricle, those of the 

 lung into the left auricle. In fishes and amphibians there is a dilatation at the 

 commencement of the aorta, the bulbus arteriosus, which is partly provided with 

 strong muscles. The reptiles possess two separate auricles, and two imperfectly 

 separated ventricles. The aorta and pulmonary artery arise separately from the 

 two latter chambers. The venous blood of the systemic and pulmonary circulations 

 flows separately into the right and left auricles, and the two streams are mixed in the 

 ventricle. In some reptiles the opening in the ventricular septum seems capable of 

 being closed. The crocodile has two quite separate ventricles. The lower vertebrates 

 have valves at the orifices of the venae cavse, which are rudimentary in birds and 

 some mammals. All birds and mammals have two completely separate auricles 

 and two separate ventricles. In the halicore the apex of the ventricles is deeply 

 cleft. Some animals have accessory hearts, e.g., the eel in its caudal vein. They 

 are very probably lymph-hearts (Robin). The veins of the wing of the bat 

 pulsate (Schiff). The lowest vertebrate, amphioxus, has no heart, but only a 

 rhythmically-contracting vessel. 



Amongst blood-glands the thymus and spleen occur throughout the vertebrata, 

 the latter being absent only in amphioxus and a few fishes. 



Amongst invertebrata a closed vascular system, with pulsatile movement, 

 occurs here and there, e.g., amongst echinodermata (star-fishes, sea-urchins, holo- 

 thurians) and the higher worms. The insects have a pulsating " dorsal vessel" as 

 the central organ of the circulation, which is a contractile tube provided with 

 valves and dilated by muscular action; the blood being propelled rhythmically 

 in one direction into the spaces which lie amongst the tissues and organs, so that 

 these animals do not possess a closed vascular system. The mollllSCa have a 

 heart with a lacunar vascular system. The cephalopods (cuttle-fish) have three 

 hearts a simple arterial heart, and two venous simple gill-hearts, each placed at 

 the base of the gills. The vessels form a completely closed circuit. The lowest 

 animals have either a pulsatile vesicle, which propels the colourless juice into the 

 tissues (infusoria), or the vascular apparatus may be entirely absent. 



105. Historical Retrospect 



The ancients held various theories regarding the movement of the blood, but they 

 knew nothing of its circulation. According to Aristotle (384 B.C.), the heart, the 



