20 GUIDES FOR VERTEBRATE DISSECTION 



On which side of the septum bulbi is the pulmo-cutaneous and 

 on which the carotid-systemic cavity? Would this imply a 

 separation of two kinds of blood as it leaves the ventricle? 



Draw the dissected heart, X4, showing these points. Then 

 read some good account of the parts and their function (e.g., 

 Gaupp, 2, pp. 247-265; Huxley and Martin, pp. 17-19). 



THE NERVOUS SYSTEM. The spinal nerves. In the dorsal 

 wall of the body cavity after removal of the viscera see several 

 pairs of white nerve trunks, the spinal nerves. Each nerve 

 makes its appearance in the midst of a grayish white mass, the 

 so-called periganglionic gland or calciferous body (in reality an 

 extension of the endolymph space of the spinal cord filled with fine 

 granules of calcium carbonate). The spinal nerves in the posterior 

 region pursue a course nearly parallel with the main axis of the 

 body; those in front are nearly at right angles to that axis. How 

 many * of these nerves do you find? Which of the anterior 

 pairs is the largest? Follow this one out, noting the region to 

 which it is distributed. Can you now explain its greater size? 

 Examine more closely and see if any of the other spinal nerves 

 send twigs to this large one, thus forming a brachial plexus. 

 Compare your results with those found by others in the laboratory 

 and see if there be any variation. 



Note that several of the posterior nerves, by means of cross- 

 branches, form a lumbo-sacral plexus. What nerves, by number, 

 enter into its constitution? Are variations found in number 

 or in arrangement of these nerves and branches? Besides 

 several smaller nerves two larger ones arising from this plexus 

 enter the hind limb, a smaller and more lateral crural nerve, 

 and a larger ischiatic nerve nearer the median line. What is 

 the distribution of each? 



Examine the spinal nerves more closely and see that each, 

 soon after leaving the periganglionic gland, gives off a small nerve, 

 ramus communicans, from its ventral surface. These rami 

 communicantes run toward the median line to join, either side 

 of the vertebral column, a longitudinal nervous structure, the 

 truncus sympatheticus. This sympathetic trunk runs on either 



* In the young there is a spinal nerve in front of the first vertebra. This 

 disappears long before the adult condition, but still the first of the per- 

 sistent nerves, issuing between vertebra? 1 and 2, must be called the second 

 of the spinal nerves. 



