186 VERTEBRATES. 



I- Spinal Nerves. Cut off the esophagus near its 

 origin, and remove the viscera. Note the absence of ribs 

 from the walls of the body cavity.. Make out ten pairs 

 of spinal nerves, as follows : 



The nerves of the first pair curve ventrally and ante- 

 riorly, and are distributed to the lower jaw. 



The nerves of the second and third pairs unite, and go 

 to the fore legs. The nerves of the second pair are large 

 and conspicuous, and extend laterally at right angles to 

 the spinal column : those of the third pair are smaller, 

 and bend anteriorly to meet those of the second. The 

 two again divide and reunite repeatedly, to form a plexus 

 (the brachial plexus). 



The nerves of the fourth, fifth, and sixth pairs extend 

 laterally and posteriorly, to be distributed to the body 

 wall. 



The nerves of the seventh, eighth, and ninth pairs go 

 to the hind legs. They unite to form a plexus (the ilio- 

 sacral plexus), which terminates in the large sciatic nerve 

 of the thigh. 



The nerves of the tenth pair are very small. They 

 may be seen close beside the posterior end of the spinal 

 column. 



II. The Brain. Remove the skin from above the 

 cranium and from one side of the head. Dissect it free 

 from the tympanic membrane with great care. Cut 

 away the bony roof of the cranium, and with a gentle 

 stream of water from a pipette wash out the soft sub- 

 stance that covers the brain and fills out the cavity. 

 Make out the following parts : 



1. A pair of conical olfactory lobes, extending forward 

 from the extreme anterior end, and tapering into the 

 olfactory nerves. 



2. Immediately behind these, the large, white, cerebral 

 hemispheres, which make up the cerebrum. 



