278 INNERVATION. [CHAP. X. 



exists between them, there can be but little doubt that fibres pass 

 from the one to the other. 



Projecting from the external and posterior part of each optic 

 thalamus, there are two small gangliform masses, similar in colour 

 and in structure to that body. These are the corpora geniculata, 

 externum and internum (fig. 71, g). Some fibres of the optic tracts 

 appear to form a connexion with them. By a transverse section 

 through either geniculate body into the substance of the thalanius, 

 the distinctness of the former may be demonstrated. 



A fissure which exists between the optic thalami is called the 

 third ventricle. Its roof is formed by the velum interpositum, one 

 of the principal internal processes of the pia mater. It contains 

 a bridge of soft grayish matter, extending from one optic thala- 

 mus to the other. This is called the middle or soft commis- 

 sure. 



The free and continuous surface of the optic thalamus and corpus 

 striatum, which projects into the anterior and middle part of each 

 lateral ventricle, is covered by a delicate epithelium, which is con- 

 tinuous with, and of the same nature as, that which lines the whole 

 interior of the ventricles. This epithelium is, probably, ciliated. 

 Beneath it we find a layer of nucleus-like particles, which also 

 extend over the whole internal surface of the ventricles. 



The pineal body, or gland as it has been miscalled, is placed 

 immediately behind the posterior extremity of the third ventricle. 

 It is a cone-shaped body, of a dark gray colour, intimately con- 

 nected with the deep surface of the velum interpositum, a process 

 of which encloses it and adheres closely to it. It rests in a groove 

 between the nates : its base is turned forwards towards the third 

 ventricle, and its apex is directed downwards and backwards. 

 No part of its base is contained in the third ventricle ; but it is 

 connected to the inner surfaces of the thalami by some fibres which 

 pass forwards from each angle of its base. These are called the 

 peduncles, or habence, of the pineal gland. A cord of transverse 

 fibres, some of which appear to be continuous with the peduncles, 

 is situated beneath the base of the body : most of these fibres are 

 connected with the posterior extremity of each thalamus, and 

 constitute what is called the posterior commissure. 



The pineal body consists principally of large nucleated vesicles, 

 and contains some tubular fibres. In a cavity which is formed 

 towards its base, is contained a mass of sabulous matter, which 

 is composed of phosphate and carbonate of lime. To this Scem- 

 mering gave the name acervulus. It is found only in subjects 



