i8o PHYLUM CHORDATA 



The ninth and tenth cranial nerves, the glossopharyngeal 

 and the vagus, respectively, arise from the side of the me- 

 dulla, back of the auditory nerve, by four roots. These unite 

 to form a single nerve, which emerges from the cranial cavity 

 by a foramen at the side of the foramen magnum. Immediately 

 back of this foramen it expands into the large jugal ganglion, 

 from which the glossopharyngeal and the vagus proceed. The 

 former passes forward to the tongue ; the latter passes back- 

 ward along the pharyngeal wall, giving off branches which 

 supply the muscles of the shoulder, the larynx, heart, lungs, and 

 stomach. 



Exercise 12. Draw the lateral aspect of the brain on a scale of 3, and 

 the cranial nerves so far as observed. 



Study the ventral surface of the brain. Cut the cranial nerves 

 and remove the brain from the skull. Put it into a dish of 

 water and study its ventral surface. Identify the olfactory lobe, 

 the hemispheres, and the structures belonging to the diencepha- 

 lon. The optic nerves will be seen issuing from the optic chiasma, 

 a structure formed by the crossing of the optic nerves on 

 the ventral side of the brain. Behind the optic chiasma is the 

 infundibulum, a large median projection which is divided into 

 a right and a left lobe, and extending from the hinder end of 

 which is a flattened body called the pituitary body. This body 

 is lodged in a depression in the floor of the cranial cavity, and 

 usually remains there after the brain is removed from the skull. 



The ventral portion of the midbrain is formed by the crura 

 cerebri, which lie beneath the optic lobe and are partly con- 

 cealed by the infundibulum. Arising from the crura near the 

 middle line may be seen the very delicate oculomotor nerves. 



The medulla oblongata is but slightly wider than the spinal 

 cord. A longitudinal groove is present in the midventral line of 

 both. 



Exercise 13, Draw the ventral aspect of the brain on a scale of 3. 



The Vascular System. This system is made up of the follow- 

 ing parts : (i) the heart, a muscular pump which is continually 



