THE PERCH 209 



lobes. The third division, or midbrain, consists of the paired 

 optic lobes, the largest part of the brain, between which and the 

 cerebrum appears a small, median, sunken area, the diencepha- 

 lon, the second division. Projecting dorsally from this division 

 is the long and slender pineal body, which is the rudiment of 

 a third optic nerve ; it may have been removed in exposing the 

 brain. Back of the optic lobes are the fourth division, the cere- 

 bellum, and the fifth division, the medulla oblongata, which is 

 continuous with the spinal cord. Note the longitudinal median 

 groove in the medulla and spinal cord, and the paired lateral 

 swellings, the restiform bodies, at the anterior end of the former. 



Exercise 11. Draw a dorsal view of the brain on a scale of 3 or 4. 



The Vascular System. This is made up of the following organs : 

 (i) the heart, which receives venous blood from the tissues and 

 forwards it to the gills ; (2) the arteries, which carry {a) venous 

 blood from the heart to the gills, and (h) arterial blood from the 

 gills to the tissues ; (3) the veins, which carry venous blood to the 

 heart ; and (4) the capillaries. 



The Venous System. Two distinct systems of veins are pres- 

 ent : one is composed of the systemic veins, which carry blood 

 directly to the heart ; the other is composed of those which carry 

 blood first to the liver, and is called the portal system. 



We shall first study the latter system. Take a fresh animal and 

 open its body cavity by a midventral incision. Cut away both 

 the right and the left body walls between the incision and the air 

 bladder. The portal system consists of a pair of intestinal veins 

 which lie alongside the intestine, a splenic vein from the spleen, 

 several gastric veins from the stomach, and a pneumatocystic 

 vein from the air bladder, all of which unite to form the single 

 large portal vein. This vein passes to the hinder surface of the 

 liver, where it breaks into branches which carry the blood to all 

 parts of that organ. 



The veins of this system are often difficult to dissect because of 

 the fat in which they usually lie embedded. First find the two 

 intestinal veins and free them from the fat. Lift up the spleen 

 and the duodenal loop in which it lies and find the point of union 



