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Avgas persicas 
through the organ, emerges on the centre of the dorsal surface, after 
leaving which, it almost immediately joins the stomach. 
As seen under a low magnification, the brain presents the appearance 
of a rounded, compact, creamy-white organ, measuring about 0'4 mm. in 
length; the maximum breadth usually exceeds the length, but becomes 
perceptibly narrower in the anterior portion. The dorsal surface is 
rounded, that portion lying anterior to the oesophagus being especially 
prominent; the ventral surface is broad and flattened, and the lateral 
margins are slightly indented between the roots of the four large trunks 
of the pedal nerves (see Text-fig. 5). 
It has already been shown that the brain is completely surrounded 
by the periganglionic blood sinus (see Part II, p. 243) and thus receives 
its blood supply direct from the heart. 
In sections of the brain, the organ is seen to be completely invested 
in a closely adherent, connective tissue sheath or neurilemma, which is 
reflected at the point of entry of the oesophagus to form a continuous 
sheath, which surrounds and separates the latter from the actual brain 
substance. The nervous tissue of the brain consists of a peripheral zone 
of ganglion cells surrounding a central mass of molecular substance, 
which is traversed in all directions by the nerve fibres which form the 
various commissures and connectives which link up the different ganglia. 
The superficial layer of ganglion cells is not of uniform thickness 
throughout its extent, being so thinned out iu places as to almost com¬ 
pletely disappear, while in other places the ganglion cells extend as 
wedge-shaped masses down into the molecular central portion, and thus 
define the ganglia from which the main nerve trunks originate. 
In spite of the extreme degree of fusion of the individual ganglia of 
which the central nerve mass is composed, it is by no means difficult to 
recognise these ganglia in sections of the brain. At first sight the brain 
appears to consist of a “ supraoesophageal ” and a “ suboesophageal 
ganglion,” which are united on either side of the oesophagus by a stout 
connective. A single pair of nerves arises from the “supraoesophageal 
ganglion,” the remainder all taking their origin from the “suboesopha¬ 
geal ganglion.” From a study of vertical and horizontal sections of the 
brain, it is seen that these “ganglia” are divided into a number of 
true ganglia from which the main nerve trunks take their origin, and 
the relations of these constituent ganglia are readily established (see 
Text-fig. 6). 
'The supraoesophageal portion of the brain is then seen to consist of 
two pairs of ganglia, the smaller pair of which is situated in the above- 
