254 THE ESSENTIALS OF HISTOLOGY. 



by a layer of white fibres, most marked next to the internal capsule, 

 fibres from which pass into the thalamus and serve to connect it with 

 the hemisphere. 



The grey matter of the thalamus (fig. 286) is partially subdivided by 

 an oblique white lamina into a smaller, inner (i), and a larger, outer, 

 nucleus (e) ; these contain a number of small scattered nerve-cells. 

 Anteriorly another portion of grey matter (a) is divided off in a similar 

 way ; this contains comparatively large nerve-cells. 



Attached to the optic thalamus below and externally are the two 

 geniculate bodies which are connected with the optic tract. Of the two 

 geniculate bodies the outer has a lamellated structure consisting of 

 alternating layers of grey and white matter. This external geniculate 

 body also has a much closer connection with the optic tract than the 

 inner ; indeed, it is doubtful whether the latter receives any fibres from 

 the tract. 



The tegmentum of the crus cerebri is prolonged below the thala- 

 mus opticus into a mass of grey substance, with longitudinally and 

 obliquely crossing white bundles, which is known under the name of 

 subthalamic substance. In it at least three parts differing from one 

 another in structure may be distinguished (see fig. 286, 1, 2, 3). 



The pineal gland, which is developed in the roof of the third 

 ventricle, is composed of a number of tubes and saccules lined and 

 sometimes almost filled with epithelium, and containing deposits of 

 earthy salts (brain-sand). These deposits may, however, occur in other 

 parts of the brain. The follicles are separated from one another by 

 vascular connective-tissue derived from the pia mater. 1 



The pituitary body is a small reddish mass which lies in the sella 

 turcica, and is connected with the third ventricle by the infundibulum. 

 It consists of two lobes, a larger anterior, and a smaller posterior. 

 The anterior lobe is originally developed as a hollow protrusion of the 

 buccal epithelium. It consists of a number of tubules, which are lined 

 by epithelium, and united by connective tissue. In some of the tubes 

 the epithelium is ciliated, and sometimes a colloid substance like that 

 occurring in the vesicles of the thyroid has been found in them. 



The posterior lobe of the pituitary body, although developed from the 

 floor of the third ventricle, contains scarcely any perceptible nervous 

 elements in the adult. It consists chiefly of vascular connective tissue. 



The membranes of the brain are similar in general structure to those 

 of the spinal cord, p. 216. The dura mater is, however, more closely 



1 In the chameleon and some other reptiles, the pineal is better developed, and 

 is connected by nerve-fibres with a rudimentary median eye of invertebrate type, 

 placed upon the upper surface of the head. 



