62 COMPARATIVE ANATOMY 



The hind-brain of the frog is a somewhat enlarged but 

 direct forward continuation of the spinal cord. It has two 

 divisions, the medulla oblongata (also called the bulb, or 

 myelencephalon) and the cerebellum (metencephalon). The 

 spinal cord, as it passes forwards into the bulb, widens out, 

 its floor becomes thicker, its roof very much thinner, and its 

 central canal widens out to form a triangular cavity, the 

 fourth ventricle, whose exceedingly thin roof is covered over 

 by a very vascular membrane. The cerebellum, relatively 

 large in many vertebrates, is represented in the frog by a 

 narrow band of nervous tissue lying transversely over the most 

 anterior part of the bulb. The mid-brain (mesencephalon) lies 

 immediately in front of the hind-brain, and in a line with it. 

 Its roof is formed by a pair of ovoid swellings, called the 

 optic lobes or corpora bigemina ; its floor, which is thick, con- 

 sists chiefly of nerve fibres running forward from the bulb to 

 the fore-brain. Each optic lobe is hollow, and its cavity com- 

 municates with a narrow passage called the Sylvian aqueduct, 

 leading from the cavity of the fourth ventricle to the cavity of 

 the fore-brain in front. This passage is sometimes called the 

 "iter." (Iter a tertio ad quartum ventrtculum.) 



The fore-brain consists of two parts, the thalamencephalon 

 behind, and the cerebral hemispheres or prosencephalon in 

 front. The side walls of the thalamencephalon are thickened 

 to form the optic thalami, but its roof and floor are thin, and 

 the former is covered over by a very vascular membrane, 

 just as is the roof of the fourth ventricle. By reason of the 

 thickening of its lateral walls, and the thinning out of its floor 

 and roof, the cavity of the thalamencephalon, known as the 

 third ventricle, is deep dorso-ventrally but narrow from side 

 to side. Its floor is produced into a conical depression, the 

 infundibulum, and its roof is produced into a hollow finger- 

 like projection, on the top of which is borne a rounded 

 vascular body usually known as the pineal gland. It appears, 

 however, that what is usually called the pineal gland is nothing 

 more than a thickened portion of the choroid plexus or 

 vascular membrane which covers this part of the head. The 

 true pineal gland is a small vesicle lying outside the skull 

 beneath the skin. In the embryo this vesicle is connected by 

 a pedicle with the roof of the thalamencephalon, but in the 

 adult frog the connection is lost, and there is no foramen 



