Page:Laboratory Manual of the Anatomy of the Rat (Hunt 1924).djvu/118

104 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, taking care not to tear the cranial nerves from the brain. Observe the connective tissue coverings (meninges) of the brain. The dura mater is the tough membrane just beneath 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 lobes are a pair of elongated structures lying between the eyes at the anterior end of the brain. The cerebrum is the large heart-shaped section lying immediately 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. Compare 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 lobes). The pineal body (epiphysis) 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 quadrigemina, two are on the right, and two are on the left. The anterior pair are the superior colliculi, the posterior are the inferior colliculi. The cerebral hemispheres are connected with each other by the corpus callosum, a broad white