48 LABORATORY MANUAL FOR ELEMENTARY ZOOLOGY 



the dorsal side. On the ventral side of the diencephalon observe the crossing 

 of the optic nerves, after their origin from the diencephalon; this crossing is 

 called the optic chiasma. Just behind the optic chiasma is a bilobed extension 

 of the floor of the diencephalon, called the inferior lobes or infundibulum; and 

 to this is attached a rounded glandular body, the hypophysis, which fits into a 

 depression in the floor of the skull, and is usually, therefore, torn off in removing 

 the brain. The hypophysis is a gland of internal secretion. 

 Draw the ventral surface of the brain. 



5. The ventricles of the brain. The brain like the spinal cord is hollow, its 

 cavity being continuous with the central canal of the cord. The cavities of the 

 brain are known as ventricles. Make a median sagittal section of the brain, 

 float it under water, and identify the ventricles. See also Holmes, Fig. 83 

 (p. 292), and Fig. 84 (p. 293). The cavity of the medulla oblongata is the largest 

 and most posterior of the ventricles; it is called the fourth ventricle and has a 

 thin vascular roof which has already been removed. From the fourth ventricle 

 a narrow passage, the iter or aqueduct of Sylvius, extends forward below the optic 

 lobes. Each optic lobe has a cavity, the optic ventricle. The ventricle of the 

 diencephalon which extends downward into the infundibulum is the third ven- 

 tricle. The first and second ventricles, also called the lateral ventricles, are 

 inside of the cerebral hemispheres, which should be cut open to see them. The 

 narrow passage which connects the lateral with the third ventricles is known as 

 the foramen of Monro. 



6. The spinal nerves (Holmes, pp. 89-91). Turn the frog ventral side up, 

 and remove all the viscera. Observe the spinal nerves passing out symmetrically 

 from the sides of the vertebral column. They arise from the cord and leave the 

 neural canal by way of openings (intervertebral foramina) between the vertebrae. 

 There are ten pairs of spinal nerves, each of which divides immediately into a 

 small dorsal branch and a larger ventral branch. It is the ventral branch which 

 one sees running on the inside of the dorsal body wall. At the point of exit of 

 each spinal nerve from the intervertebral foramen is a white mass, the calcareous 

 body, which surrounds and conceals a ganglion. 



Identify and observe the course of each of the spinal nerves. The first is 

 quite small; it arises between the first and second vertebrae and innervates the 

 tongue and muscles of the hyoid. The second is a large stout nerve which 

 innervates the muscles of the fore limb. It is joined by branches of the first 

 and third spinal nerves, and all of these together form a network which is called 

 the brachial plexus. The presence of such a plexus indicates the compound 

 origin of the muscles of the limb. The fourth, fifth, and sixth nerves are small 

 and pass somewhat obliquely backward to supply the skin and muscles of the 

 body wall. The seventh, eighth, ninth, and tenth nerves arise close together, 

 run almost directly backward, and are united with one another by cross branches 

 to form the sciatic plexus from which nerves go to the muscles and skin of the 



