The Brain Of Petromyzon - An Attempt To Define The Primitive Functional Divisions Of The Central Nervous System (1902) (Paperback)

Purchase of this book includes free trial access to www.million-books.com where you can read more than a million books for free. This is an OCR edition with typos. Excerpt from book: with the ventral surface of the medulla and surrounding the roots of the XII nerve. B. The Mid Brain. a. Tectum opticum (Figs. 2, 12, 13, 14, 30). As already noted, the caudal part of the tectum presents the characteristic appearance of the bi-lobed fish tectum. the two sides being connected by the dorsal decussation. In its cephalic part the two lobes become separated, there is no dorsal decussation, and the roof is a choroid plexus. The plexus grows wider cephalad as the lateral walls recede and the tectum disappears at the level of the posterior commissure. The tectum presents a well defined grey zone adjoining the cavity made up of from three to six closely packed layers of cells, and a fiber zone which makes up more than three-fourths of the thickness of the wall. The fiber zone contains a large number of cells scattered irregularly through it. Upon the outer surface of the tectum throughout its whole extent there are isolated cells at short intervals which are pyramidal or balloon shaped, with the large end against the limitans externa and the smaller end penetrating the fiber zone. The tectum has a much larger number of cells in proportion to its whole volume or the volume of its fiber zone than there are in Acipenser. The cells of the tectum are not so well differentiated as in Acipenser, but some of the same types are to be recognized. The cell and fiber zones are not distinguished in Golgi preparations and for the study of the minute structure the cells may be divided into two groups, deep and superficial cells. The deep cells are vertical or horizontal. The vertical cells have ovoid or spindle shaped cell bodies which are usually within the cell zone and sometimes have central processes reaching the cavity. Their dendrites are few in number and rise towar...