A LABORATORY OUTLINE OF NEURO-ANATOMY 363 



45. The fiber tracts, of which the white substance is composed, cannot be distin- 

 guished in the normal adult cord. They can be recognized from differences in the degree 

 of their myelination in fetal cords (p. 112 and Fig. 79) and in preparations showing 

 degeneration resulting from disease or injury in various parts of the nervous system 

 (p. 105; Figs. 75, 76). From such preparations as are available for this purpose and 

 from your reading (pp. 95-112) form a clear conception of the origin, course, and ter- 

 mination of each of the fiber tracts. 



46. Indicate the location of each of these tracts in the outline drawing of the 

 cervical portion of the spinal cord, entering the ascending tracts and the ventral cortico- 

 spinal tract on the right side, and all of the descending tracts except the ventral cortico- 

 spinal tract on the left side. Why should the ventral and lateral corticospinal tracts 

 be indicated on opposite sides of the cord? Wax crayons should be used to give the 

 several tracts a differential coloring. Use the following color scheme: 



Somatic afferent tracts: 



Proprioceptive yellow. 



Exteroceptive blue. 

 Somatic motor tracts: 



Corticospinal tracts red. 



Rubrospinal tract brown. 

 All other tracts black. 



47. The fasciculus cuneatus and fasciculus gracilis should be colored yellow and 

 then dotted over with blue to indicate that while the proprioceptive fibers predominate, 

 there are also some exteroceptive fibers in these tracts. 



THE BRAIN STEM 



48. Now take the human brain and identify all of its principal divisions. Dissect 

 out the arterial circle of Willis, and identify the branches of the internal carotid, ver- 

 tebral, and basilar arteries. Read about the blood-supply and meninges of the brain 

 in your text-book of anatomy. Identify all of the cranial nerves (Fig. 86). 



49. Examine again the cerebellar peduncles in the three specimens of the sheep's 

 brain (Figs. 87, 91). Now remove the cerebellum from the previously intact sheep's 

 brain. Cut through the peduncles on both sides of the brain as far as possible from 

 the pons and medulla, sacrificing the cerebellum to some extent in order to leave as 

 much of the peduncles as possible attached to the brain stem. Be careful not to damage 

 the anterior medullary velum and the tela chorioidea which lie under cover of the 

 cerebellum (Fig. 84). In the same way remove the cerebellum from the human brain. 



50. Study the roof of the fourth ventricle in both the human and the sheep's brain 

 (pp. 128, 129 and Figs. 84, 90, 154). Examine the chorioid plexus of the fourth ven- 

 tricle. Note the line of attachment of the tela chorioidea. Tear this membrane away. 

 The torn edge which remains attached to the medulla is the taenia of the fourth ventricle 

 (Figs. 89, 90). Study the attachments of the anterior medullary velum. The decus- 

 sation of the trochlear nerve within the velum can easily be seen in the sheep. Remove 

 this membrane. The floor of the fourth ventricle is now fully exposed. 



51. Remove the pia mater from the brain stem, carefully cutting around the roots 

 of the cranial nerves with a sharp-pointed knife to prevent these nerves being torn 

 away from the brain when this membrane is removed. 



