846 



THE NERVOUS SYSTEM 



of the third ventricle. Though it develops as a diverticulum of that portion of 

 the anterior primarj^ vesicle which gives origin to the thalamencephalon, it is 

 wholly a non-nervous structure, other than the sympathetic fibres which enter it 

 for the supply of its blood-vessels. 



It consists of a dense capsule of fibrous tissue (pia mater) from which numerous septa pass 

 inward, dividing the interior into a number of intercommunicating compartments filled with 

 epithehal (ependymal) cells of the same origin as the ependyma lining the ventricles and aque- 

 duct below. Among these ceUs are frequently found small accretions (brain-sand, acervulus 

 cerebri), consistmg of mixed phosphates of hme, magnesia, and ammonia and carbonates of 

 lime. The compartments form a closed system. In function the epiphysis ranks as one of the 

 glands of internal secretion of the body, and it is often referred to as the 'pineal gland.' How- 

 ever, it is perhaps functionless in man. 



Fig. 667. — Mesi.\l Section of Entire Brain, showing Mesial Surface op Diencephalon 

 AND Telencephalon. (After Henle.) 



Massa 

 Hypothalamic sulcus intermedia 

 Posterior commissure 

 Epiphysis 



Splemum of corpus 



callosum 

 Lamina 

 quadngemina 



Interventricular foramen (Monroi) 

 / Sulcus of corpus callosum 



Septum pellucidum 

 Anterior comimssure 

 Sub-callosal 

 gyrus 



Aqueduct of 

 cerebrum (Sylvii) 

 Anterior medullary 

 velum 



Genu of corpus 

 ^ callosum 



Rostrum of corpus 



callosum 

 Lamina terminalis 



Cerebellum 



Fourth ventricle 



Medulla 



Pons Mammil- Tuber 

 lary body cinereum 



\ Optic nerve 



Optic chiasma 



Apparently arising from the base of the epiphysis, but having practically 

 nothing to do with it, are the striae medullares of the thalamus (striae pineales, 

 pedunculi conarii, taenia thalami, habenulae). These are two thin bands of white 

 substance which extend from under the epiphysis anteriorly upon the thalamus, 

 along the superior border of each lateral wall of the third ventricle, and thus form 

 the boundaries between the superior and mesial surfaces of each thalamus. 



They have been called the habenulce, from their relation to the habenular nucleus, situated 

 in the mesial grey substance at their inferior ends. They are continuous across the mid-line 

 in the habenular commissure, just below the neck of the epiphysis, and between it and the pos- 

 terior (lorebral commissure, or, rather the superior part of the latter (figs. 631, 665). It will be 

 seen below that each liabcnula contains olfactory fibers from the fornix, the anterior perfor- 

 ated substance and the septum pellucidum, as well as fibres out of the thalamus, and that most 

 of its fibres terminate in the habenular nucleus. 



The ventro-lateral surface of the thalamencephalon is continuous into the 

 hypothalamic tegmental region, the upward (jontinuation of the tegmental grey 

 substance of the mesencephalon. It is also adjacent to a portion of the internal 

 capsule. Both these relationsiiips, as well as the fibre connections of the dien- 

 cephalon with the structures above and below it, are deferred until the discussion 

 of the internal structure of the prosencephalon. 



The mesial surface of the (lienc('))halon (fig. GO?), allows a better view of the 

 shape and relations of the third ventricle. I^elow the line of the massa inter- 

 media the ventricle is usually somewhat wider than it is along the upper margins of 



