282 Dynamic Theory. 



disappears by the fourth month of foetal life. The Gorilla and Chim- 

 panzee ordinarily have only eight carpals as in man, but in the Orang 

 and most of the lower apes and in some of the Rodents there are always 

 nine. So that when we have this reversion it is to a type below the 

 Chimpanzee or Gorilla. 



Every naturalist now admits that the various stages of development 

 of an animal as well as its specialized parts are found to correspond with 

 permanent conditions of animals lower in the scale. A good illustra- 

 tion of this is seen in the development of the human heart and blood- 

 vessels. In the early stages of development we have a heart with a 

 single cavity connected with a vessel at each end as in ascidians ; later 

 on the blood-vessels consist of a series of arches which go to the gills or 

 bronchial clefts as in fishes and amphibia, while the heart consists of 

 two chambers separated by valves, and is placed forward in the neck. 

 The gill arches now partly disappear, and though the circulation still 

 remains single as in Reptiles, the heart cavities are beginning to be sep- 

 arated into two distinct systems. Soon a double circulation is required 

 by a complete separation of the heart into right and left. The right 

 heart propels the venous, and the left the arterial blood. At this 

 period the condition is identical with that of birds ; at last the true 

 mammalian type of heart and blood-vessels develops and remains per- 

 manent. The arrangement of the blood-vessels going to and from the 

 heart varies considerably in different mammals. In man the rule is for 

 the great artery, carrying the blood from the heart to the general sys- 



FIG. 113. Normal aortic arch in man. 

 re. Right carotid artery. lc. Left carotid artery. 



rs. Right subclavian artery. Is. Left subclavian artery. 

 IS i. Innominate artery. 



tern, to give off three main branches, named, the 

 innominate, left carotid and left subclavian. 

 These are distributed to the head and the two 

 arms ; the main vessel, or aorta, curves down- 

 ward and distributes blood to the trunk and lower 

 extremities. These branches are now known to 

 be derived from certain of the original gill-arches 

 which persist, ( See figs. 27, 29 ) and when any 

 FIG. us. variation in their arrangement takes place it al- 



ways occurs in the line of some of these gill-arches ; that is, some of 

 arches persist which usually are obliterated. Nearly all the variations 

 occuring in these large vessels in man are found to be the regular con- 

 dition in animals lower in the scale ; for instance, sometimes only two 

 branches are given off instead of three ; each of these again dividing 

 into two, one for the head and one for the arm of that side. (See fig. 

 114 B. ) This is the usual arrangement in the bat, porpoise and dol- 



