218 DISSECTION OF THE DOG 



This is a convenient opportunity to study the association of the hippo- 

 campal fissure of the exterior with the hippocampus in the interior. It will 

 be observed that the two coincide in position ; indeed, the fissure 

 causes the elevation of the hippocampus. Between the hippocampal 

 fissure and the fimbria hippocampi is a narrow, somewhat corrugated 

 strip, the fascia dentata hippocampi. 



Tela chorioidea ventrictjli tertii. — The choroid tela of the third 

 ventricle is a triangular double fold of pia mater containing a layer of arachnoid 

 and some blood-vessels. It is interposed between the fornix and part of the 

 hippocampus on the one side and the thalami on the other. The apex of 

 the triangle lies at the foramen of Monro. Each of its sides is bounded by 

 the choroid plexus of the central part of the lateral ventricle. At its base the 

 tela divides into its two component sheets of pia mater continuous with the 

 pia covering the surface of the brain generally. 



The cleft into which the choroid tela and the choroid plexuses of the inferior 

 cornua of the lateral ventricles are insinuated, is known as the transverse 

 cerebral fissure (fissura transversa cerebri). 



Dissection. — Seize the apex of the choroid tela with forceps and strip it 

 from the surface of the thalami upon which it lies. As this is done 

 the cleft-like third ventricle will be exposed. 



Thalamus. — The thalamus is an oval mass of grey matter separated from 

 the caudate nucleus by an oblique groove containing a narrow white band, the 

 stria terminalis. The dorsal surface of the thalamus is convex in the main 

 and bounded medially by a prominent border formed by a longitudinal ridge, 

 white in colour, known as the medullary stria (stria medullaris). The anterior 

 end of the stria appears to join one of the columns of the fornix, while pos- 

 teriorly it is apparently connected with the pineal body. The medial surfaces 

 of the two thalami are flattened and form the lateral walls of the narrow 

 third ventricle. Crossing the ventricle and uniting the two thalami is a thick, 

 rounded grey bridge of tissue, the intermediate mass (massa intermedia). The 

 lateral and ventral surfaces of the thalamus cannot be demonstrated at present , 

 as they are in connection ■«ith other parts of the brain. The lateral surface 

 is in contact with a band of white matter known as the internal capsule. The 

 ventral surface rests on the dorsal or tegmental part of the cerebral peduncle 

 (the hypothalamus). 



Corpus pineale. — In the middle line where the posterior part of the two 

 thalami are in contact, the small and inconspicuous pineal body will be found. 

 The base of the body contains one of the recesses of the third ventricle. 



Ventriculxjs tertitjs. — The third ventricle is the narrow space between 

 the two thalami. The choroid tela forms the roof of the ventricle, while its 

 floor corresponds to the tuber cinereum, mammillary body and the posterior 

 perforated substance. Anteriorly the cavity is bounded by the terminal lamina, 



