Dissection of Sheep^s Brain, 161 



ing of the Aorta is close to the Septum Ventricularum, and is 

 guarded by three Sigmoid valves, larger and stronger through- 

 out than those in the Pulmonary artery : from the two 

 anterior Sinuses of Yalsava arise the Coronary arteries. 



The bone of the Heart (cf. supra, Circulation) though gen- 

 erally present in Buminants does not appear in the Sheep till 

 in advanced years* : consequently is seldom found in the 

 Heart of a Sheep killed for food. 



II. SHEEP'S BRAm. 



N.B. It icill he icell to place a fresh Braki in spirit for a 

 week at least ])rior to commencing the dissection, in order that it 

 may he thoroughly hardened. 



Examination of the Exterior. The whole exterior of 

 the Brain will be seen to be closely invested by a membrane, 

 called the Pia Mater, which dij)s into all the sulci between 

 the convolutions, and forms a medium in which the arteries 

 and veins may ramify. Besides covering the exterior of the 

 brain, this membrane sends processes into the interior, in 

 which the vessels that supply the walls of the enclosed space 

 can be conveyed ; thus one process penetrates into the Cere- 

 brum below the Corpus Callosum and is termed the Velum 

 Interpositum ; and two vascular fringes project into the fourth 

 ventricle, known as the choroid plexuses of that cavity. 



* Milne Edivards. Lecons siir la PAysto%2c,t.iii.,p.492, note, " Chez le Mou- 

 ton il ne parait se former qu'^ un age avance. [Duvernoy, Lecons d'Anatomie 

 Comparee de Cuvier, t. vi. p. 292. F. S. Leuckart. Bemerkungen (Meckel's 

 Deutsches Archiv,, t. vi. p. 136).] 



