226 DISSECTION OF THE DOG 



the original vesicle from which the hind-brain developed. Consequently it 

 is in communication with the central canal of the spinal cord at the one end 

 and with the cavities of the rest of the brain at the other. The opening of 

 the central canal into the ventricle has already been commented upon. If the 

 anterior medullary velum be raised the communication with the aqueduct of 

 Sylvius — and through this with the third ventricle — will be revealed. 



In outline the cavity is rhomboidal ; hence the name fossa rhomboidea as 

 applied to its floor. From the narrow caudal part — calamus scriptorius — it 

 widens to the level of the place where the restiform bodies turn up into the 

 cerebellum. Here the transverse diameter is materially increased by the 

 presence of lateral recesses (recessus laterales), which curve round the caudal 

 aspect of the restiform bodies. In front of the recesses, the ventricle narrows 

 to the entrance of the aqueduct of Sylvius. 



The roof or dorsal wall of the ventricle is formed by the two medullary 

 vela and the thin pial continuation of the posterior velum. Seen in sagittal 

 section, the roof is somewhat tent-like, being continued into a recess, the 

 fastigium, between the vermis attachments of the two vela. The roof, moreover, 

 is rendered irregular on a level with the lateral recesses by an invagina- 

 tion produced by the choroid plexus (plexus chorioideus ventriculi quarti). 

 That part of the plexus which is related to the recesses can readily be seen 

 in the intact brain as a vascular tuft occupying an angular depression formed 

 by the cerebellum and medulla oblongata. 



The membranous roof is defective at the extremity of the lateral recess, 

 where a small irregular opening, the lateral aperture (apertura lateralis ven- 

 triculi quarti), places the cavity of the ventricle in communication with the 

 subarachnoid space. 



The floor of the ventricle is formed by the medulla and pons. Divided into 

 two lateral halves by a median longitudinal groove, the floor is further sub- 

 divided into areas. A longitudinal limiting groove (sulcus limitans) forms the 

 lateral boundary of an elongated and narrow medial area. The limiting 

 groove is quite shallow, but deepens a little at two places known as the anterior 

 and posterior fonece. A prominent medial eminence (eminentia medialis) occurs 

 about the middle of the length of the strip demarcated by the median and 

 limiting grooves. A triangular area, the hypoglossal trigone (trigonum nervi 

 hypoglossi) intervenes between the posterior fovea and the median groove, 

 while lateral to it is the grey ala cinerea. An elevated acoustic area (area 

 acustica) forms a conspicuous object lateral to the limiting sulcus and opposite 

 the recess of the ventricle. 



Dissection. — It is now necessary to remove the remains of the muscles 

 attached to the vertebral column, the ribs, the pelvis, and the occipital 



