334 HISTOLOGY. 



into the medulla. Branches from the plexus in the capsule descend between 

 the cell groups of the cortex and terminate on the surface of the cells in the 

 two outer zones; they do not enter between the separate cells. The plexus 

 in the zona reticularis is more abundant, but here also only groups of cells 

 are supplied. In the medulla the nerves are extraordinarily abundant and 

 each cell is surrounded by fibers. Groups of sympathetic ganglion cells 

 may be found, but these rarely occur in the cortex. A part of the nerves 

 terminate in the walls of the vessels. 



In the vicinity of the ductus deferens and in the broad ligament of the 

 uterus, suprarenal bodies may occur, consisting only of cortical substance. 

 Groups of chromamne cells have been found in relation with the paro- 

 ophoron and paradidymis. 



BRAIN AND SENSE ORGANS. 



BRAIN. 

 DEVELOPMENT AND GENERAL ANATOMY. 



In a previous section the formation of the medullary tube from the 

 primitive ectoderm has been described, and it has been stated that the 

 posterior portion of the tube becomes the spinal cord and that the anterior 

 portion forms the brain. In a human embryo of 4.0 mm., the tube still 

 opens freely through a large anterior neuropore, the extent of its connection 

 with the -epidermal ectoderm being indicated in Fig. 390, A. The tube 

 has become bent in two places; the posterior or neck bend is near the 

 junction of the cord and brain, the line of separation between which must 

 be arbitrarily drawn both in the embryo and in the adult; the anterior or 

 head bend occurs in a part of the tube called the mid-brain (mesencephalori) . 

 In front of the mid-brain is the fore-brain (prosencephalori) and behind it is 

 the hind-brain (rhombencephalori) . The entire brain is therefore divided 

 into fore-brain, mid-brain, and hind-brain. In an early stage the fore- 

 brain produces two lateral outpocketings, one on either side, called the 

 optic vesicles. Each expands distally to form the retina of an eye and its 

 connection with the fore-brain becomes reduced to a sknder stalk. In 

 later stages the depression on the inner wall of the brain wSich marks the 

 position of the stalk is called the optic recess. 



The hind-brain soon becomes rhomboid or kite-shaped as seen from 

 its dorsal surface. This is due to a widening of the cavity of the medullary 

 tube; its lateral walls spread apart and the roof plate becomes thin and 

 transparent. The dilated cavity of the hind-brain is called the fourth 

 ventricle; the cavity of the mid-brain in the adult is a slender passage 

 called the aqueduct [of Sylvius]; it becomes vertically expanded in the 



