8 LABORATORY MANUAL FOR ELEMENTARY ZOOLOGY 



The general pleuroperitoneal membrane in the region of the heart leaves the 

 body wall in the median dorsal line and passes ventrally on each side of the peri- 

 cardial sac and in inseparable contact with it; the two sides then meet ventrally 

 below the heart and form a double membrane which extends vertically from the 

 pericardial sac to the ventral median line under the sternum. This vertical 

 membrane also supports the liver, the large reddish-brown organ around and 

 posterior to the heart, and this part of it is therefore known as the suspensory 

 or falciform ligament of the liver. Follow the suspensory ligament posteriorly 

 along the median ventral line and note that it supports the large anterior 

 abdominal vein which was mentioned in the preceding section. Follow along 

 the median ventral line and at the posterior end of the coelome locate the median 

 ligament of the bladder, a mesentery which attaches the urinary bladder, a thin- 

 walled, often shriveled sac, to the ventral body wall. 



There was originally in the embryo, a complete double-walled mesentery 

 running from the median dorsal line to the median ventral line, and inclosing the 

 viscera between its two walls. The coelome was thus divided into two entirely 

 separated halves. The portion of this mesentery between the digestive tract 

 and the dorsal wall is called the dorsal mesentery, and that from the digestive 

 tract to the ventral wall, the ventral mesentery. The dorsal mesentery, as will 

 be seen shortly, is still intact in the adult frog, but the ventral mesentery has 

 entirely disappeared, except for certain remnants already mentioned the 

 suspensory ligament of the liver, and the ligaments of the bladder, and certain 

 ligaments running from the liver to the intestine, which will be described below. 



Cut through the ventral mesenteries, and pin out the body wall, making 

 crosscuts if necessary so that the viscera will be fully exposed. 



E. GENERAL INTERNAL STRUCTURE 



The frog is made up of a number of definite structures called organs, each of 

 which has a definite function to perform. Ah 1 of the organs which aid in per- 

 forming the same function are grouped together as a system or tract. The organs 

 constituting one system may be all alike or may be different among themselves. 

 In general in a complex animal there are ten systems: skin and its derivatives, 

 skeletal, muscular, digestive, circulatory, respiratory, excretory, reproductive, nervous, 

 and sensory systems. To this list, there should probably be added, in the case 

 of vertebrates, an eleventh, composed of a number of glands, which were originally 

 derived from the other systems, but have lost connection with them and have 

 taken on peculiar but extremely important functions. This group of glands is 

 spoken of collectively as the glands of internal secretion, also as cryptoretic or 

 endocrinous organs. Attention has already been called to the muscular and 

 skeletal systems; the other systems will now be described briefly and will be 

 studied in detail later (Holmes, chap, iv, pp. 73-80). 



