The pineal body is a small reddish structure, which is adherent to the under surface of the velum interpositum, so that it is often torn away in removing the choroid plexus and velum interpositum. It is connected by two peduncles of white substance with the optic thalami in front. In the adult the structure of the pineal gland is very indefinite, but it is often partly cystic, and it usually contains calcareous particles, or brain sand.

At an early period of development the pineal gland contains saccules and follicles lined with cylindrical epithelium, and this structure is retained in adult life in some animals. According to recent observations the pineal gland is an aborted or rudimentary median eye, the Pineal eye, which in the reptilia still shows sufficient structure to be recognizable. The eye is of the invertebrate type having the rods and cones in front of the expansion of the nerve instead of behind it as in the vertebrata. These observations may perhaps explain the occurrence of tumours with epithelium, cartilage, and other structures in the pineal gland.

The pineal gland is occasionally the seat of tumours. There are simple enlargements from hyperplasia, sometimes with increase of the brain sand constituting a psammoma. There are also cysts replacing the gland.

Bulky tumours of complex structure have been observed originating in the pineal gland, sometimes attaining to the size of a small apple. Weigert has described a teratoma which measured an inch and three-eighths in diameter, and contained hairs, hair-follicles, sebaceous glands, cartilage, fat, smooth muscle, cylindrical epithelium, and perhaps nerves. Falkson and the author have recorded cases to which the name chondro-adeno-sarcoma might be given, in which the bulk of the tumour was sarcomatous, but associated with cartilage, glandular structures, and cysts (see Fig. 349), and Turner has described one somewhat similar in structure but without cartilage.

From the position of the pineal gland these tumours are specially liable to distend the third ventricle, and they are also apt to press on the optic thalami, peduncles and corpora quadrigemina (see Fig. 350).