356 THE VERTEBRATE BODY 



ments at the time of copulation. The persistence of a large inguinal 

 canal in the body wall of a man often results in inguinal hernia when 

 a fold of the intestine is pushed down through this canal. 



In a woman the ostium of the oviduct is closely connected to the 

 ovary, so that the eggs which break out of the ovary usually drop right 

 into it rather than going first into the body cavity and then finding their 

 way to the ostium. Sometimes they do fall free in the body cavity and 

 in certain rare cases have even been fertilized there and started embry- 

 onic development. The uterus of a woman is the place where the em- 

 bryo develops rather than just a temporary storage place for eggs as in 

 the frog. The human embryo derives nourishment from the uterus of 

 its mother through its placenta and, therefore, it is not necessary for the 

 human egg to store a large amount of food in the form of yolk as is the 

 case in the frog egg. In a woman the single uterus opens to the vagina, 

 which leads to the outside, while in the frog the two uteri open into the 

 cloaca. 



There is no seasonal cycle of reproduction in human beings as there 

 is in frogs, but the activity of this system is somewhat stimulated in the 

 spring of the year so the well-known quotation, "In spring, a young 

 man's fancy . . . ," has some biological basis. There is a monthly cycle 

 of ovulation in a woman that somewhat corresponds to the yearly cycle 

 of ovulation in the frog, but a woman normally releases only one egg 

 from her ovaries during ovulation while the frog may release hundreds. 



The Endocrine System. There is a series of secretory glands located 

 in various regions of the chordate body that have no ducts to carry their 

 products. For this reason they are sometimes called ductless or endo- 

 crine glands. These glands secrete vital substances called hormones that 

 are absorbed by the blood and lymph flowing through the glands and 

 carried to cells all over the body. The hormones are regulators that 

 govern many of the vital body reactions. The frog possesses endocrine 

 glands and hormones which are similar to those found in man. How- 

 ever, most of the research on this body system has been done on the 

 higher vertebrates including man. Therefore, it will be best for us to 

 make our study of this system on the glands and hormones as they are 

 found in the human body. 



The thyroid glands are endocrines located in the neck on either side 

 of the trachea connected across the front by a narrow isthmus of thyroid 

 tissue. They secrete a hormone, thyroxin, that regulates the rate of 

 metabolism in the cells of the body. A person with an excessive secre- 

 tion of this hormone, hyperthyroidism, will have a high rate of metabo- 

 lism. This results in a number of symptoms characteristic of this condi- 

 tion. With a high rate of metabolism there is a high rate of heat pro- 



