180 MAMMALIA. 



proportion of membrane. This plexus may be well seen in a calf a few days old, 

 without any dissection, before the fat has begun to accumulate ; and in the baboon, 

 monkey, jaguar, fox, and sow, by just separating the attachments of cellular membrane 

 at the side of the bladder and rectum ; its demonstration was also easily made in a 

 very young ass, but in another, presumed to be at least a year old, the dissection 

 was tedious, as the nerves were covered by, and intermixed with, a dense membrane. 

 It is probable that the medullary portions of the nerves of this plexus are intrinsically 

 similar, but that they have a thicker membrane or covering from the surrounding parts, 

 not only in the same animal at different periods of life, but under the particular 

 circumstances to which the organs supplied by them have been subjected. These 

 nerves will therefore be satisfactorily exhibited in some animals rather than in others, 

 or even in man, for when they are imbedded in a thick membrane, they cannot be so 

 easily nor entirely separated. It is necessary, in examinations with the microscope, 

 that this variation should be borne in mind, not only with respect to the nerves of this 

 particular plexus, but to those in other situations. 



In the porpoise, the upper portion of the sympathetic, as far as the middle of the 

 thorax, is very much developed, but the lower part is not in the same degree. The 

 thoracic portion has large and fleshy ganglia, probably on account of its supply to the 

 vascular rete, branches from which are sent to mix with and terminate amongst this 

 structure ; and it is a most interesting fact, for confirming the opinion entertained of 

 the great influence the sympathetic nerve has in directing the circulation of the blood. 

 It is large in proportion to the spinal cord, and particularly its ganglia in the thorax. 

 The intercostal spinal nerves are the same as in mammalia generally. If the sympa- 

 thetic arose from the spinal cord, it need not have been enlarged for this change of the 

 bloodvessels, but the spinal nerves would have answered the same purpose. 



The sympathetic is often a slender and delicate nerve, therefore its safe position 

 has been carefully chosen. In the neck of man and some animals it is confined to the 

 spine by strong cellular membrane, but in others it is joined with the trunk of the par 

 vagum. In the thorax and abdomen, and pelvis, it is commodiously fixed at the sides 

 of the vertebrae, but more or less inclining towards the middle of the lumbar, according 



