104 ANATOMY OF THE RAT 



of the cranium and immersing the animal in the formalin 

 solution. Dissect away the muscles on one side of the 

 head, preserving all nerves found. Remove the dorsal and 

 posterior walls, and one lateral wall of the cranium, tak- 

 ing care not to tear the cranial nerves from the brain. Ob- 

 serve the connective tissue coverings {meninges) of the 

 brain. The dura mater is the tough membrane just be- 

 neath the bone. Remove it. The pia mater is the delicate 

 membrane, containing blood vessels, resting directly upon 

 the brain. Between these is the arachnoid layer. 



The olfactory lo^es are a pair of elongated structures 

 lying between the eyes at the anterior end of the brain. 

 The cerehrum is the large heart-shaped section lying im- 

 mediately behind the olfactory lobes. It consists mostly 

 of two lateral hemispheres separated by the longitudinal 

 fissure. The cerebellum is the ovoid structure posterior to 

 the cerebrum. It includes the median vermis and the right 

 and left hemispheres. Observe the transverse furrows on 

 both vermis and lobes. The medulla oblongata succeeds 

 the cerebellum and joins the spinal cord posteriorly. Com- 

 pare the cerebral hemispheres of the rat with those of 

 the dog, cat, man, or other higher mammal. The rat's 

 cerebrum lacks the grooves (sulci) and ridges (gyri) found 

 in these other forms. 



Clear away the blood vessels on the surface of the brain. 

 The longitudinal fissure widens posteriorly, exposing the 

 corpora quadrigemina {optic lo^es). The pineal body {epi- 

 physis) is the globular structure at the posterior end of the 

 longitudinal fissure. Carefully press aside the hemispheres 

 and observe that of the four lobes of the corpora quadri- 

 gemina, two are on the right, and two are on the left. The 

 anterior pair are the superior colUculi, the posterior are the 

 inferior colUculi. The cerebral hemispheres are connected 

 with each other by the corpus callosum, a broad white 



