Integrative Systems 101 



tory activity, but activity of sweat-glands is increased by sympathetic 

 stimulation. Evidence that the sympathetic nerve-endings on sweat- 

 glands produce acetylcholine instead of sympathin goes far toward 

 explaining this anomaly. While the nerves to the sweat-glands are 

 anatomically sympathetic, they are functionally parasympathetic. 



Autonomic activities, with one or two possible exceptions (e.g., 

 action of the ciliary muscles of the eye), are involuntary. Within the 

 autonomic system itself there are no obvious structural barriers to 

 volition. Its apparent inability to traverse rami communicantes and 

 operate in visceral territory is probably due, not to any obstacles in 

 the visceral field, but rather to lack of appropriate connections between 

 autonomic reflex circuits and the dominating nervous centers in the 

 dorsal wall of the telencephalon. However that may be, it seems quite 

 clear that the animal is fortunate in having a well-trained and efficient 

 autonomic servant to do the daily routine drudgery of attending to the 

 intricate details of visceral activity, leaving the somatic master free to 

 devote himself to outside affairs. Under ordinary circumstances, most 

 visceral activities do not even rise into consciousness. But any derange- 

 ment, such as results when unacceptable food is thrust upon a stomach, 

 may be attended by consequences which rise acutely into conscious- 

 ness. Even so, the somatic animal is unable to do anything about it 

 directly. The visceral mechanism has its own ways of meeting emer- 

 gencies, and its responses are usually corrective in their nature. 



Meninges 



The central nervous organs (brain and cord) are invested by con- 

 nective-tissue membranes, the meninges, which serve for their pro- 

 tection and nourishment. In fishes, the cranium and the vertebrae are 

 lined by compact perichondrium or periosteum (connective-tissue 

 membranes investing surfaces of cartilage or bone, respectively), 

 between which and the brain or cord the connective tissue is loose, 

 except where it comes in contact with the central nervous organs. 

 There it becomes the more compact and highly vascular meninx 

 primitiva. 



Two such meninges occur in amphibians, a highly vascular pia 

 mater primitiva next to the brain and cord and, outside this, a 

 thicker dura mater. The wide space between dura mater and 

 periosteum is bridged by connective-tissue strands. 



In mammals three meninges are differentiated (Fig. 162). Inner- 

 most is the pia mater, thin and highly vascular, from which connec- 

 tive-tissue processes growing into brain and cord carry in blood-vessels 

 and support the nervous tissue. Outside this is the arachnoid which, 

 as its name suggests, is a delicate weblike tissue. Only its outer layer 



