SPECIAL PHYSIOLOGY. 



born, each begins to respire by the lungs. A modification of the cir- 

 culation then becomes necessary, to connect it with, and adapt it to 

 the newly-employed respiratory organs: the allantoid or placental 

 circulations become arrested ; the allantois is withered, and the pla- 

 centa is first detached from the maternal system, and afterwards from 

 the young animal, near the umbilicus or navel. From the umbilicus 

 inwards, the umbilical vein shrinks on its coagulated contents, as far 

 as the portal vein, and forms the future round ligament of the liver. 

 From the portal vein to the inferior vena cava, the venous channel, 

 called the ductus venosvs, likewise contracts on its coagulated blood, 

 and is then obliterated, remaining only as a fibrous cord. The fora- 

 men ovale in the auricular septum becomes completely closed. The 

 ductus arteriosus, connecting the pulmonary artery with the arch of 

 the aorta, the primitive ductus Botalli, also contracts on its coagu- 

 lated contents, and is soon converted into a fibrous cord; whilst the 

 right and left divisions of the pulmonary artery leading to the lungs 

 become enlarged, and convey a far larger quantity of blood than be- 

 fore ; the pulmonary veins, which bring back the blood from the lungs 

 to the left auricle, are, at the same time, proportionally enlarged. 

 Lastly, the portions of the hypogastric arteries which pass upwards by 

 the sides of the bladder and urachus, to issue at the umbilicus as the 

 umbilical arteries, likewise shrink and become obliterated. By these 

 changes, which are accomplished within three or four days, the circu- 

 lation acquires its permanent condition, or what is called the complete 

 double circulation; the venous blood, returned from the whole body, 

 is propelled by the right auricle, exclusively into the right ventricle, 

 and from thence, through the pulmonary artery and its right and left 

 branches, entirely into the lungs. From these it is returned, arte- 

 rialized, to the left auricle, is exclusively delivered into the left ven- 

 tricle, and is thence propelled through the ascending and descending 

 aorta and their branches, on to every part of the body. To complete 

 these changes, the left ventricle, which now performs the whole work 

 of the systemic circulation, speedily acquires a greater thickness of 

 its walls than the right ventricle. 



DEVELOPMENT OF THE TISSUES. 



Animal and Vegetable Cells. 



Whilst the forms and relations of the organs are evolved in the 

 manner just detailed, their component tissues are undergoing develop- 

 ment. These tissues, with their complex structure and composition, 

 are gradually produced from simple, and originally identical, anatom- 

 ical elements. They commence, like the organs which they build up, 

 in the early periods of the formation of the embryo, originating di- 

 rectly from, or through the agency of, nucleated cells of the blasto- 

 derm, held together by an intermediate matrix or blastema (/2Arfro<r, 

 blastos, a germ). These cells are themselves the internal offspring of 

 the contents of two cells the germ cell and the sperm cell. The for- 

 mer, or nidal cell, fertilized by the latter, produces a brood of uniform 



