38 NERYOUS SYSTEM AKD GENERAL SENSATION. 



primitive ova ; they are constituted after the general type of 

 cellular formations, although they have more of the character 

 of solid bodies than of true cells with fluid contents.*] 



[ 84. The general form and distribution of the nervous sys- 

 tem of animal life is shown in the annexed plate (fig. 19), 

 which represents the cerebro-spinal system, and the course of 

 the principal nerves in man. At a are seen the two hemi- 

 spheres of the cerebrum ; at b those of the cerebellum ; and at 

 c the spinal cord. The principal motory nerve, passing to the 

 muscles of the face, is seen at d ; and at e, the brachial plexus 

 formed by the interlacing of five spinal nerves, destined to give 

 off branches to the upper extremities. The principal of these 

 are, the median nerve, f, which passes down the arm ; the 

 ulriar nerve, g, which passes round the inner condyle of the 

 humerus, is distributed to the integument and muscles, and 

 sends terminal twigs to the ring, and fourth fingers ; the 

 internal cutaneous nerve, h ; and the radial and muscular 

 nerves, i, which are in like manner distributed to the integu- 

 ment and muscles of the fore-arm, hand, and fingers. From 

 the spinal cord are given off" the intercostal nerves, j, which, 

 escaping through the holes formed in the spinal column, pass 

 between the ribs, and are lost in the skin and muscles of 

 the trunk. The lumbar plexus, k, sends nerves to the front of 

 the thigh and leg; the sacral plexus, /, gives origin to the 

 principal nerves of the lower extremities. The great sciatic 

 nerve the largest nerve in the body proceeds down the back 

 of the thigh, and at the ham divides into the tibial nerve, m y 

 the external peroneal, or fibular nerve, n, and the external 

 saphenous nerve, o. 



[ 85. THE BRAIN is a compound organ, enclosed in the 

 skull, and surrounded by three membranes : these are, the dura- 

 mater, the external or fibrous, the pia-mater, the middle, or 

 vascular; and the arachnoid, the internal or serous. These mem- 

 branes are prolonged into the canal of the spinal column for 

 lodging the cord, and invest in like manner this central portion 

 of the nervous system. Figure 20 will serve to give the 

 student a general idea of the different parts which compose 

 the brain. It represents a vertical section of the cerebrum, a ; 

 the cerebellum, d ; the medulla oblongata, e ; and shews the 



* Professor Wagner's Elements of Physiology, p. 464, et seg. 



