116 A TEXTBOOK OF PHYSIOLOGY 



In the mollusca we find definite blood -channels, the larger of which, 

 in the cephalopods, have well-developed muscular walls, and act as 

 pumps driving by wave-like contractions the contained fluid before 

 them. 



In the lobster there is developed a heart. It is endowed with a 

 rhythmic activity of its own, and forces the blood through a system 

 of larger vessels the arteries to smaller frailer vessels the capil- 

 laries and thence to open spaces between the masses of connective 

 tissue the lacunae. From these lacunas the blood is returned to 

 the heart by another system of channels the veins. 



In insects the circulatory mechanism is simple. A dorsal pump 

 the heart forces the blood through a vessel which runs in the 

 median line from one end of the body to the other into large sinuses 

 and spaces; from these it is returned to the heart. In the limbs 

 are placed accessory hearts, which force the blood to then- 

 extremities. 



In the vertebrates the evolution of the circulatory system is carried 

 to the highest point. Blood is pumped from a well-differentiated 

 strong muscular heart, by means of an arterial system with well- 

 marked muscular and elastic walls, into a capillary system the walls 

 of which are formed by a single layer of endotheliuin. The lacunar 

 system still persists in part, for in certain organs, such as the spleen, 

 the capillaries are not closed vessels, but open into the tissue spaces. 

 From the capillaries the blood is returned by larger channels the 

 veins to the heart. 



In Amphioxus the blood vascular system is still of the primitive 

 lacunar type This lowest vertebrate possesses two hearts a dorsal 

 heart, driving arterial blood to the system, and a ventral one, which 

 is termed the " respiratory heart," sending blood to the gills. In 

 fishes the heart is single, and essentially respiratory in function, 

 propelling blood to the gills, thence to the aorta and to the system 

 generally, and back again to the heart. 



In the amphibia there are two auricles and one ventricle ; in reptiles 

 two auricles and a partial separation of the ventricle into two. It is 

 only in the birds and mammalia that the two systems become quite 

 distinct two auricles and .two vyentrieles the right auricle and 

 ventricle forming the respiratory system, the left auricle and ventricle 

 the systemic. This evolution has, however, been carried out on 

 quite a different plan in the two hearts, the bird's heart differing in 

 many points from the mammalian. 



In man the heart is about equal in size to a closed fist, measuring 

 about 5 inches long, 3 inches wide, and weighing in the adult about 

 300 grammes, or 046 per cent, of the body weight. In the new-born 

 baby it weighs about 24 grammes, 0-76 per cent, of the body weight. 

 The average weight of the male and female heart is almost the same. 

 The volume is estimated by filling the cavities with wax, to be 100 to 

 130 c.c. for each auricle, and 150 to 200 c.c. for each ventricle. 



The auricles have much thinner walls than the ventricles. The 

 muscle of the auricles consists of a circular layer common to both, 



