VASCULAR SYSTEM icy 



Since on the authority of Bresslau this line has been brought 

 about by a coalescence of the marsupial sacs, Klaatsch is justi- 

 fied in defining it as a marsupial character. 



Characteristic of many of the lower animals (ruminants, 

 pigs, carnivora), and entirely absent in man and the apes, are 

 the serous cutaneous glands which secrete a watery fluid and 

 thus keep the outer terminal part of the nose in a moist state. 



The anal sac in the carnivora and the inguinal fold in 

 sheep, with their tubulous and aciniform glands, correspond to 

 the axillary and perineal glands in man, in so far as they emit 

 a secretion with an offensive odour, and first become function- 

 ally active in the adult, thus standing, apparently, in close re- 

 lation with the reproductive functions. 



IV. Vascular System. 



In common with the other vertebrates man possesses a com- 

 plete system of closed canals, or vessels (see Fig. 57), contain- 

 ing the nutritious juices of the body, while in the invertebrates, 

 with the exception of the higher worms and some members of 

 the arthropoda, this system is absent. Another leading dis- 

 tinction between the vertebrates and invertebrates is the situa- 

 tion of the heart ; in the latter it is formed from a dorsal 

 vascular trunk, or part of such, whereas in the former the heart 

 is developed from a section of a ventral vascular trunk. The 

 tunicata are the only invertebrates possessing a ventrally placed 

 heart. Moreover, the vertebrata did not attain to the pos- 

 session of a heart simultaneously with that of a vertebral 

 column, for in the headless amphioxus, which stands at the very 

 base of the class, all the greater vascular trunks are contractile, 

 and the circulation of the blood is carried on from several points 

 and not confined to any special one. In the Craniata, on the 

 other hand, the heart forms a distinct organ for regulating the 

 flow of the nutritious fluid in the blood and lymph vessels. 



The heart of the Craniata has been developed from a simple 

 tube which became gradually divided into two parts : a posterior 

 part, the auricle, and an anterior, the ventricle, both being en- 

 veloped in the pericardium which invests the whole. 



In the fishes there is still only one auricle and ventricle, 



